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THE  ABSORPTION  SPECTRA  OF  SOLUTIONS 


OF 


CERTAIN  SALTS  OF  COBALT,  NICKEL,  COPPER,  IRON,  CHROMIUM,  NEODYMIUM, 

PRASEODYMIUM,  AND  ERBIUM  IN  WATER,  METHYL  ALCOHOL, 

ETHYL  ALCOHOL,  AND  ACETONE,  AND  IN  MIXTURES 

OF  WATER  WITH  THE  OTHER  SOLVENTS 


BY 


HAEEY  G.  JONES  and  JOHN  A.  ANDEESON 


^     UNIVERSITY     ^A. 

I*  0CT25  lg09^1 


WASHINGTON,  D.  C. 
Published  by  the  Carnegie  Institution  op  Washington 

1909 


CAENEGIE  INSTITUTION  OF  WASHINGTON 
Publication  No.  110 


PRESS  OF  J.  B.  LIPPmCOTT  COMPANY 
PHILADELPHIA 


QC 
431 


PREFACE. 

This  investigation,  which  is  a  continuation  of  the  work  of  Jones  and 
Uhler  on  the  absorption  spectra  of  solutions  (Carnegie  Publication  No.  60), 
was  carried  out  with  the  aid  of  a  Grant  generously  awarded  by  the  Carnegie 
Institution.  Salts  of  the  following  metals  were  brought  within  its  scope: 
Cobalt,  nickel,  copper,  iron,  chromium,  neodymium,  praseodymium,  and 
erbium;  and  about  1,200  solutions  were  studied. 

Spectrograms  of  salts  of  these  metals  in  water,  varying  the  concen- 
tration but  keeping  the  total  amount  of  coloring  matter  in  the  path  of  the 
beam  of  light  constant,  were  obtained.  In  a  similar  manner  spectrograms 
were  made,  keeping  the  total  number  of  molecules  in  the  path  of  the  beam 
of  light  constant.  The  effect  of  such  dehydrating  agents  as  calcium  chloride 
and  aluminium  chloride,  on  absorption  in  solution,  was  also  studied. 

A  large  number  of  salts  were  rendered  anhydrous  and  dissolved  in 
methyl  alcohol,  ethyl  alcohol,  and  acetone.  The  concentration  of  the  solu- 
tions in  these  solvents  was  varied,  but  the  total  amount  of  coloring  matter 
in  the  path  of  the  light  was  kept  constant.  Finally,  water  was  added  to 
the  non-aqueous  solutions,  and  its  effect  on  absorption  determined. 

Perhaps  the  most  striking  result  was  obtained  with  anhydrous  neodym- 
ium chloride  in  alcoholic  solutions,  to  which  small,  increasing  amounts  of 
water  were  added.  Here  entirely  new  bands  appeared  on  the  addition  of  a 
small  amount  of  water. 

An  ample  supply  of  the  salts  of  neodymium  and  praseodymium  was 
furnished  us  with  their  characteristic  liberality  by  the  Welsbach  Light 
Company,  and  our  thanks  are  especially  due  to  their  chemist,  Dr.  H.  S. 
Miner. 

We  accept  this  opportunity  to  extend  our  thanks  to  Prof.  J.  S.  Ames, 
who  has  placed  at  our  disposal  the  ideal  conditions  under  which  this  inves- 
tigation was  carried  out. 

Harry  C.  Jones. 

m 


CONTENTS. 


CHAPTER  I.  PAQB 

Introductory 1 

Apparatus 6 

Photographic  Material,  etc 7 

Sources  of  Light 8 

Making  a  Spectrogram 9 

CHAPTER  II. 

Salts  op  Cobalt 11 

Cobalt  Chloride  in  Water — Beer's  Law 13 

Cobalt  Chloride  in  Water — Number  of  Ions  in  the  Path  of  the  Beam  of  Light 

Constant 16 

Cobalt  Chloride  in  Water — Molecules  Constant 15 

Cobalt  Chloride  in  Methyl  Alcohol — Beer's  Law 16 

Cobalt  Chloride  in  Ethyl  Alcohol — Beer's  Law 17 

Cobalt  Chloride  in  Acetone — Beer's  Law 18 

Cobalt  Chloride  in  Methyl  Alcohol  with  Water 19 

Cobalt  Chloride  in  Ethyl  Alcohol  with  Water 20 

Cobalt  Chloride  in  Acetone  with  Water 21 

Cobalt  Bromide  in  Water — Beer's  Law 22 

Cobalt  Bromide  in  Water — Molecules  Constant 22 

Cobalt  Bromide  with  Calcium  Bromide 23 

Cobalt  Bromide  in  Methyl  Alcohol — Beer's  Law 25 

Cobalt  Bromide  in  Ethyl  Alcohol — Beer's  Law 25 

Cobalt  Bromide  in  Acetone — Beer's  Law 26 

Cobalt  Bromide  in  Methyl  Alcohol  with  Water 27 

Cobalt  Bromide  in  Ethyl  Alcohol  with  Water 27 

Cobalt  Bromide  in  Acetone  with  Water 28 

Cobalt  Nitrate  in  Water — Beer's  Law 29 

Cobalt  Nitrate  in  Water — Molecules  Constant 30 

Cobalt  Sulphate  in  Water — Beer's  Law 31 

Cobalt  Sulphocyanate  in  Water — Beer's  Law 32 

Cobalt  Sulphocyanate  in  Water — Molecules  Constant 33 

Cobalt  Acetate  in  Water — Beer's  Law 34 

General  Summary  of  Results  with  Cobalt  Salts 35 

CHAPTER  III. 

Samts  op  Nickel 39 

Nickel  Chloride  in  Water — Beer's  Law 39 

Nickel  Chloride  in  Water — Ions  Constant 40 

Nickel  Chloride  in  Water — Molecules  Constant 41 

Nickel  Chloride  in  Water  with  Calcium  and  Alimiinium  Chlorides 41 

Nickel  Sulphate  in  Water — Beer's  Law 42 

Nickel  Acetate  in  Water — Beer's  Law 43 

CHAPTER  IV. 

Salts  of  Copper 46 

Copper  Chloride  in  Water — Beer's  Law 46 

Copper  Chloride  in  Water — Molecules  Constant 46 

Copper  Chloride  in  Methyl  Alcohol — Beer's  Law 46 

Copper  Chloride  in  Ethyl  Alcohol — Beer's  Law 47 

Copper  Chloride  in  Acetone — Beer's  Law 48 

V 


VI  CONTENTS. 

CHAPTER  IV.  Sai/ts  op  Copper. — Continued.  page 

Copper  Chloride  in  Methyl  Alcohol  with  Water 49 

Copper  Chloride  in  Ethyl  Alcohol  with  Water 50 

Copper  Chloride  in  Acetone  with  Water 60 

Copper  Bromide  in  Water — Beer's  Law 51 

Copper  Bromide  in  Water — Molecules  Constant 52 

Copper  Bromide  in  Methyl  Alcohol — Beer's  Law 62 

Copper  Bromide  in  Ethyl  Alcohol — Beer's  Law 53 

Copper  Bromide  in  Methyl  Alcohol  with  Water 64 

Copper  Bromide  in  Ethyl  Alcohol  with  Water 64 

Copper  Nitrate  in  Water — Beer's  Law 55 

Copper  Nitrate  in  Water — ^Molecules  Constant 56 

CHAPTER  V. 

Sai/ts  of  Iron 59 

Ferric  Chloride  in  Water — Beer's  Law 59 

Ferric  Chloride  in  Water — ^Molecules  Constant 59 

Ferric  Chloride  with  Calcium  Chloride 60 

Ferric  Chloride  with  Aluminium  Chloride 60 

Ferric  Chloride  in  Methyl  Alcohol — Beer's  Law 61 

Ferric  Chloride  in  Ethyl  Alcohol — Beer's  Law 61 

Ferric  Chloride  in  Acetone — Beer's  Law 62 

CHAPTER  VI. 

Salts  of  CHROMixna 63 

Chromium  Chloride  in  Water — Beer's  Law 64 

Chromiimi  Chloride  in  Water — Molecules  Constant 65 

Chromiiun  Chloride  with  Calcium  Chloride  and  Aluminium  Chloride 65 

Chromium  Nitrate  in  Water — Beer's  Law 66 

Chromium  Nitrate  in  Water — Molecules  Constant 67 

CHAPTER  VII. 

Salts  of  Neodymium,  Praseodymium,  and  Erbium 68 

Preparation  of  Anhydrous  Salts 71 

Neodymiiun  Chloride  in  Water — Beer's  Law 72 

Neodymium  Chloride  in  Water — ^Molecules  Constant 76 

Neodymium  Chloride  in  Water  with  Calcium  Chloride  and  with  Aliuninium 

Chloride 77 

Neodymium  Chloride  in  Methyl  Alcohol — Beer's  Law 77 

Neodymium  Chloride  in  Ethyl  Alcohol — Beer's  Law 78 

Neodymium  Chloride  in  Mixtures  of  Methyl  Alcohol  and  Water 79 

Neodymium  Chloride  in  Ethyl  Alcohol  with  Water 83 

Neodymium  Chloride — Anhydrous 84 

Neodymium  Bromide  in  Water — Beer's  Law 86 

Neodymium  Nitrate  in  Water — Beer's  Law 87 

Neodymium  Nitrate  in  Water — Molecules  Constant 91 

Neodjmaium  Nitrate  in  Methyl  Alcohol — Beer's  Law 91 

Neodymium  Nitrate  in  Ethyl  Alcohol — Beer's  Law 92 

Neodymium  Nitrate  in  Acetone — Beer's  Law 92 

Neodymium  Nitrate  in  Mixtures  of  Methyl  Alcohol  and  Water 93 

Neodymium  Nitrate  in  Mixtures  of  Acetone  and  Water 94 

Praseodymium  Chloride  in  Water — Beer's  Law 94 

Praseodymium  Chloride  in  Mixtures  of  the  Alcohols  and  Water 95 

Praseodymium  Nitrate  in  Water — Beer's  Law 96 

Erbium  Chloride  in  Water — Beer's  Law 97 

Erbiiim  Nitrate  in  Water — Beer's  Law 97 

CHAPTER  VIII. 

Summary  and  Conclusions 100 


THE  ABSORPTION  SPECTRA  OF  SOLUTIONS. 


BY 

HAEEY  C.  JONES  and  JOHN  A.  AISDERSON. 


CHAPTER  I. 
INTRODUCTORY. 

The  work  on  absorption  spectra  of  solutions,  of  which  this  forms  a 
part,  was  begun  in  the  fall  of  1905  and  continued  during  the  year  1905-6 
by  Jones  and  Uhler.  The  results  obtained  are  given  and  discussed  in 
"Hydrates  in  Aqueous  Solutions,"  by  H.  C.  Jones,  Publication  No.  60  of 
the  Carnegie  Institution  of  Washington.  The  first  part  of  that  work  had 
to  do  with  the  effect  of  adding  various  dehydrating  agents  to  solutions  of 
colored  salts;  the  second  part  dealt  with  the  change  in  the  absorption 
spectra  produced  by  adding  water  to  non-aqueous  solutions.  In  the 
latter  phase  of  the  work,  however,  the  concentration  of  the  colored  salt 
was  varied  in  the  solutions  used  in  making  any  one  spectrogram;  but  as 
the  change  in  the  spectrum  observed  was  always  the  same  qualitatively 
as  would  be  expected  from  the  change  in  concentration,  it  was  deemed 
advisable  to  carry  out  again  this  part  of  the  earlier  work,  keeping  the 
concentration  of  the  colored  salt  constant. 

The  salts  used  in  the  previous  investigation  were:  Cobalt  chloride, 
copper  chloride,  and  copper  bromide.  In  this  investigation  cobalt  bromide 
was  added  to  the  above  list  of  compounds.  The  resulting  spectrograms 
show  the  same  general  change  in  the  absorption  as  in  the  earlier  work, 
thus  proving  that  this  was  not  due  to  change  in  concentration,  but  to 
some  action  of  the  water  added. 

If  we  assume  that  the  absorption  of  light  is  due  to  vibrating,  charged 
particles,  or  electrons,  which  are  associated  with  ions,  molecules,  or  groups 
of  one  or  both  of  these,  it  is  natural  to  expect  that  the  character  of  the 
absorption  will,  in  general,  depend  upon  the  nature  of  the  system  with 
which  the  vibrating,  charged  particle  is  associated.  In  what  follows,  the 
system  made  up  of  the  charged  particle  and  whatever  it  is  associated 
with  will  be  spoken  of  simply  as  the  "absorber." 

The  simplest  case  of  an  absorbing  solution  would  be  one  containing 
only  one  kind  of  "absorber,"  and  we  shall  speak  of  it  as  a  "simple"  ab- 
sorbing solution.  Such  a  solution  does  not  in  all  probability  exist,  but  is 
perhaps  closely  approached  in  such  cases  as  the  very  dilute  solutions  of 
the  salts  of  permanganic  acid  studied  by  Ostwald  and  others.  In  such  a 
jsoliition  the  absorption  of  lig^^  of  ?  gi^g"  wavf^-length  woiild  |7p~RTTTrply 
juv^pnrf.innfl.1  f.Q  the  number  of  absorbers  in  the  path  of  the  lipht  (Beer's 
law);  ftndjf  thft  abfjprhprs  ftrp  not  f-hanged  bv  adding  more  of  the  sol- 
ventjjt  follows  at  once  that  if  the  product  of  concentration  and  thickneaa, 
oTiayer  of  the  solution  is  kept  constant,  the  absorption  will  be  unchangexL 
^so,  it  we  have  a  solution  containing  several  kinds  of  absorbers,  each 
acting  independently  of  the  others,  the  same  statement  would  be  true; 

1 


2  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

in  other  words,  the  absorption  spectrum  of  a  mixture  of  simple  solutions 
will  be  the  sum  of  the  absorption  spectra  of  the  constituents. 

If  the  concentration  is  very  great,  the  absorbers  may  be  so  close 
together  that  they  cease  to  act  independently  of  each  other;  hence,  even 
if  the  solution  is  a  simple  absorbing  one,  Beer's  law  may  cease  to  hold  if 
the  concentration  is  very  great.  In  general,  however,  we  may  say  that 
for  simple  absorbing  solutions  or  mixtures  of  these.  Beer's  law  will  hold. 

Actual  solutions  always  differ  more  or  less  from  the  ideal  simple  solu- 
tions on  account  of  the  changes  produced  by  dilution.  These  changes 
are  due  to  association,  dissociation,  and  solvation. 

The  molecules  of  the  dissolved  substance  may  combine  with  each 
other,  forming  more  or  less  complex  aggregates,  each  of  which  will,  in 
general,  have  its  own  peculiar  power  of  absorbing  light.  The  composi- 
tion of  the  aggregates  will  depend  upon  temperature  and  concentration, 
and  hence,  if  a  solution  containing  such  aggregates  is  diluted,  we  should 
expect  to  find  deviations  from  Beer's  law  even  if  the  temperature  is  kept 
constant. 

The  molecules  of  a  great  number  of  substances  when  dissolved  disso- 
ciate into  two  or  more  ions,  the  amount  of  dissociation  depending  upon 
the  concentration.  It  is  to  be  expected  that  the  absorption  of  the  ions 
into  which  a  molecule  dissociates  will  be  different  from  that  of  the  mole- 
cule itself,  and,  consequently,  on  diluting  an  electrolyte  we  should  expect 
to  find  deviations  from  Beer's  law,  unless  the  solution  is  so  dilute  that  it 
may  be  considered  as  completely  dissociated. 

We  may  also  have  various  combinations  of  molecules,  aggregates  of 
molecules  or  ions,  not  only  with  each  other,  but  also  with  the  molecules 
of  the  solvent,  the  nature  of  which  will  depend  both  on  temperature  and 
concentration,  and  each  of  which  may  have  a  different  power  of  absorbing 
light.  If  a  simple,  colored  electrolyte  like  cobalt  chloride,  for  example, 
is  dissolved  in  water,  we  see  at  once  what  a  complicated  system  the  solu- 
tion is;  and  it  is  not  surprising  that,  in  spite  of  the  great  amount  of  work 
which  has  already  been  done  on  this  one  salt  alone,  we  are  still  far  from 
able  to  give  a  satisfactory  account  of  its  absorption  spectrum. 

A  satisfactory  account  of  the  absorption  of  any  salt  in  solution 
requires  a  knowledge  of  the  kinds  of  absorbers  the  salt  forms,  and  the 
amount  of  each  for  any  given  set  of  conditions.  Given  this  knowledge, 
it  would  be  necessary  to  determine  what  would  be  the  absorption  spec- 
trum if  the  solution  contained  only  one  kind  of  absorber.  This  could 
be  done  as  follows:  Suppose  a  salt  in  solution  gives  rise  to  the  absorbers 
A,  B,  C,  D,  E,  etc.,  the  amount  of  each  of  which  is  supposed  to  be  known 
under  all  conditions  of  temperature,  pressure,  concentration,  etc.  Vary 
the  conditions  in  such  a  way  that  all  of  the  absorbers  except,  say,  A  are 
kept  constant,  and  note  the  change  in  the  spectrum;  this  change  is  due 
to  A  alone,  since  by  hypothesis  no  other  is  varied.  Repeat,  only  keep  all 
except  B  constant,  and  so  on.  By  this  process  of  elimination  we  should 
eventually  arrive  at  a  complete  knowledge  of  the  absorption  due  to  each 
absorber,  and  could  hence  predict  beforehand  exactly  what  would  be  the 
absorption  of  any  solution  whatever  of  the  salt  in  question.     Unfortu- 


INTRODUCTORY.  3 

nately  our  knowledge  of  the  parts  formed  when  a  salt  is  dissolved  is  still 
very  vague.  We  have  methods  for  measuring  dissociation,  so  we  may 
regard  the  number  of  ions  and  the  number  of  undissociated  molecules  as 
known  for  different  conditions. 

Regarding  aggregates  and  solvates,  however,  our  knowledge  is  very 
general,  indeed.  Determinations  of  molecular  weights  give  some  idea  con- 
cerning the  existence  or  non-existence  of  aggregates,  and  the  methods  of 
Jones  and  others  furnish  similar  ideas  about  solvates;  but  the  knowledge 
gained  thus  far  is  not  definite  enough  to  enable  us  to  perform  experiments 
along  the  lines  indicated  above.  A  great  deal  can,  however,  be  learned 
not  only  about  absorption,  but  also  about  the  nature  of  solutions,  by  the 
study  of  absorption  spectra  under  conditions  which  are  varied  as  much  as 
possible. 

The  methods  commonly  employed  are: 

(1)  To  keep  the  concentration  constant,  varying  the  depth  of  the  cell 
and  photographing  the  spectra  of  successive  depths  one  beneath  the  other, 
so  that  the  complete  spectrogram  gives  an  idea  of  the  intensity  of  absorp- 
tion for  the  different  regions  of  the  spectrum,  besides  locating  the  absorp- 
tion bands. 

(2)  To  keep  the  depth  of  cell  constant  and  varying  the  concentration. 
The  results  here  should  be  identical  with  those  obtained  by  keeping  concen- 
tration constant  and  varying  the  depth  of  cell,  provided  the  solution  is  of 
such  a  nature  that  Beer's  law  holds;  in  general,  however,  the  two  methods 
give  quite  different  results,  owing  to  the  change  in  the  nature  of  the 
absorbers  produced  by  dilution. 

Another  method  is  that  followed  in  the  present  work,  namely,  to  vary 
both  depth  of  layer  and  concentration  in  such  a  manner  that  the  product 
of  the  two  remains  constant.  If  the  nature  of  the  absorbers  is  not 
changed  by  dilution,  this  method  leaves  the  number  of  absorbers  in  the 
path  of  the  beam  of  light  constant,  and  hence  the  spectrum  for  successive 
solutions  should  be  identical;  or,  what  amounts  to  the  same  thing,  the 
width  of  the  absorption  bands  as  shown  by  the  spectrogram  should  remain 
constant  throughout.  Any  deviation  from  Beer's  law  would  at  once  be 
seen  by  the  bands  changing  in  width  or  position  as  the  concentration  is 
varied. 

A  modification  of  this  method,  also  employed  in  the  present  work,  is  to 
vary  the  depth  and  concentration  in  such  a  manner  that  the  total  num- 
ber of  ions,  or  the  total  number  of  undissociated  molecules  in  the  path  of 
the  beam  of  light  remains  constant.  This  is  easily  done  as  follows:  Let 
the  concentration  be  denoted  by  c,  the  ratio  of  the  number  of  dissociated 
molecules  to  the  total  number  put  into  solution  by  x,  the  depth  of  solu- 
tion used  by  d;  the  number  of  ions  in  a  cubic  centimeter  of  the  solution 
is  then  proportional  to  ex,  and  the  number  of  undissociated  molecules  to 
c(l  —  x).  To  keep  the  number  of  ions  in  the  path  of  the  beam  of  light  con- 
stant it  is  only  necessary  to  keep  the  product  cxd  constant;  and  to  keep 
the  number  of  undissociated  molecules  constant  the  product  c{l—x)d  must 
remain  constant.  If  the  successive  depths  of  solution  to  be  used  have 
been  fixed  arbitrarily,  the  concentrations  are  determined  in  the  follow- 


4  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

ing  way:  From  tables  giving  the  values  of  x  for  different  values  of  c  the 
products  ex  and  c(l — x)  are  calculated  for  all  values  of  c.  Two  curves 
are  then  plotted,  one  between  ex  and  c,  the  other  between  c(l — x)  and  c. 
Letting  q,  x^,  and  d^  represent  the  values  of  c,  x,  and  d  for  the  greatest 
concentration  to  be  used,  the  values  for  any  other  concentration  being 
represented  by  the  same  letters  without  subscripts,  we  have,  respectively, 

CiX^d^  =  cxd  Ci(l—Xi)di  =  c{l—x)d 
or, 

c^x^di                                        Ci(l  —  Xi)d 
cx^-j-  c(l-a:)  = ^ 

The  terms  on  the  right  are  both  known,  and  hence  the  products  ex  and 
c{\—x)  for  any  chosen  value  of  d  are  known;  and  from  the  two  curves  the 
corresponding  values  of  c  may  be  read  off  directly. 

Since  more  is  known  about  dissociation  than  about  association  or 
solvation,  it  is  only  natural  to  try  to  see  whether  the  observed  changes 
in  absorption  can  be  explained  by  dissociation  alone.  If  dissociation  does 
not  sufl&ce,  then  we  must  conclude  that  other  factors  come  in.  The  pres- 
ent work  is  devoted  largely  to  a  study  of  the  absorption  spectra  of  a  large 
number  of  salts  from  the  standpoint  of  dissociation. 

Let  us  consider  for  a  moment  the  kind  of  evidence  obtained  by  the 
methods  outlined,  and  what  conclusions  may  be  drawn  from  them.  Con- 
sider first  the  possibility  that  an  absorption  band  does  not  change  in 
width  or  position  with  concentration,  when  the  product  of  depth  of  layer 
and  concentration  is  kept  constant.  The  simplest  explanation  is  that 
the  absorption  of  the  molecule  and  of  the  ions  into  which  it  breaks  down 
on  dissociation  is  the  same.  An  excellent  example  of  this  type  is  fur- 
nished by  the  ultra-violet  band  of  nickel  sulphate  (see  Plate  28) ;  also  by 
the  more  dilute  solutions  of  neodymium  and  praseodymium  chloride. 

Let  us  now  consider  cases  where  we  have  deviations  from  Beer's  law. 
Take  first  the  possibility  that  an  absorption  band  widens  with  dilution, 
when  the  product  of  concentration  and  depth  of  layer  is  kept  constant. 
This  would  indicate  either  that  the  band  is  due  to  ions,  or  that  the  ions 
have  stronger  absorption  in  the  region  considered  than  the  undissociated 
molecules.  By  making  a  series  of  exposures,  keeping  the  number  of  ions 
in  the  path  of  the  beam  of  light  constant,  we  can  decide  between  the  two 
possible  explanations.  If  the  band  now  remains  of  constant  width  and 
position,  it  is  most  likely  due  to  ions  alone;  if  it  narrows  on  dilution,  the 
ions  have  stronger  absorbing  power  than  the  undissociated  molecules; 
while  if  the  band  should  widen  with  decrease  in  concentration,  dissocia- 
tion would  in  no  way  suffice  to  explain  it. 

The  other  case  is  where  the  band  narrows  with  dilution,  when  the 
product  of  concentration  and  depth  of  layer  is  kept  constant.  If  disso- 
ciation can  account  for  this  we  must  either  have  the  undissociated  mole- 
cules absorbing'^more  strongly  than  the  ions  formed  from  them,  or  else 
the  ions'not  absorbing  at  all.  In  the  former  case  the  band  should  widen 
with  dilution  when  "undissociated  molecules"  in  the  path  of  the  light  are 


INTR(y5)UCT0RY.  5 

kept  constant,  while  in  the  latter  case  the  band  would  remain  constant  in 
width  and  position  under  the  same  conditions.  If  the  band  narrows  with 
decrease  in  concentration  when  the  number  of  undissociated  molecules  is 
kept  constant,  dissociation  alone  can  not  possibly  explain  the  facts.  An 
example  of  this  case  is  furnished  by  the  ultra-violet  absorption  of  most 
copper  salts  in  aqueous  solution. 

The  case  often  met  with  in  the  present  work  where  a  band  narrows 
with  dilution  when  the  product  of  concentration  and  depth  of  layer  is 
kept  constant,  but  widens  when  the  number  of  undissociated  molecules 
is  kept  constant,  deserves  further  consideration,  as  dissociation  may  or 
may  not  be  able  to  account  for  it,  depending  on  the  nature  of  the  change 
in  the  band.  This  was  discussed  by  E.  Miiller,  who  showed  that  by  meas- 
urement of  the  extinction  coefficient  at  various  concentrations,  using  a 
spectrophotometer,  we  may  determine  whether  Beer's  law  holds  for  each  one 
of  the  three  absorbers  in  a  solution  of  an  electrolyte,  even  when  these  have 
quite  different  powers  of  absorption.  Work  on  this  point  is  now  in  prog- 
ress for  the  solutions  showing  the  effect  just  spoken  of,  and  the  results  will 
be  published  shortly. 

Another  interesting  method  of  studying  absorption  spectra  is  to  keep 
both  concentration  and  depth  of  layer  constant,  and  to  vary  the  tem- 
perature. Much  work  of  this  kind  has  already  been  done  by  Hartley  and 
others,  and  some  work  is  now  in  progress  in  this  laboratory.  As  is  well 
known,  changing  the  temperature  has  only  a  small  effect  on  the  dissocia- 
tion, hence  we  may  say  that  when  the  temperature  is  varied  the  disso- 
ciation remains  roughly  constant.  The  spectrum  of  some  solutions,  how- 
ever, undergoes  very  great  changes;  for  example,  solutions  of  cobalt 
chloride  which  are  red  at  room  temperatures  become  blue  when  the 
temperature  is  elevated  sufficiently;  and  according  to  Donnan  and  Bassett 
solutions  of  the  same  salt  in  ethyl  alcohol,  which  at  ordinary  temperatures 
are  blue,  turn  red  when  cooled  down  to  — 75°  C.  This  effect,  which  involves 
the  appearance  or  disappearance  of  a  complicated  set  of  absorption  bands 
in  the  red,  can  evidently  not  be  accounted  for  by  dissociation,  since  it  takes 
place  when  the  dissociation  is  known  to  change  but  very  slightly. 

In  the  present  work  considerable  attention  has  been  given  to  solutions 
in  non-aqueous  solvents,  such  as  methyl  alcohol,  ethyl  alcohol,  and  ace- 
tone, as  well  as  to  solutions  in  mixtures  of  these  solvents  with  water. 
In  this  part  of  the  work  great  care  has  been  taken  to  have  both  the  salts 
and  solvents  as  free  from  water  as  possible,  and  this  is  of  fundamental 
importance,  as  is  shown  by  the  spectrograms  of  solutions  of  neodymium 
chloride  in  mixtures  of  alcohol  and  water,  where  it  appears  that  the  change 
in  the  spectrum  produced  by  1  per  cent  or  less  of  water  may  easily  be 
detected. 

The  question  of  the  effect  of  the  solvent  comes  up  in  this  connection, 
and  this  in  turn  involves  the  more  general  question  as  to  the  condition 
of  a  molecule  of  a  dissolved  substance  in  any  solvent  whatsoever.  Does 
it  form  some  kind  of  a  compound  with  the  solvent,  or  does  it  move  about 
freely  in  the  solvent  without  materially  changing  its  nature?    Kundt's 


b  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

law,  which  states  that  the  effect  of  the  solvent  upon  absorption  is  simply 
to  shift  the  position  of  the  absorption  bands,  these  being  located  nearer 
the  region  of  long  wave-lengths  the  greater  the  dispersion  of  the  solvent, 
seems  to  imply  that  the  molecule  of  the  dissolved  substance  exists  in  the 
free  state  in  solutions.  If  this  view  be  taken,  then  the  effect  of  the  sol- 
vent should  not  be  very  great,  especially  if  solutions  in  solvents  having 
similar  optical  properties  are  compared.  If,  on  the  other  hand,  a  salt 
when  it  goes  into  solution  always  forms  some  sort  of  a  compound  with 
the  solvent,  we  might  expect  to  find  radical  differences  in  the  absorption. 
It  is  evident  that  a  study  of  the  absorption  spectra  of  a  salt  when 
dissolved  in  various  solvents  ought  to  bring  out  many  points  of  interest 
as  bearing  upon  the  question  of  the  nature  of  solutions. 

APPARATUS. 

For  visual  examination  of  solutions  a  small,  direct-vision,  grating, 
pocket  spectroscope  was  always  at  hand,  and  was  found  very  useful  for 
the  purpose  of  examining  solutions  in  order  to  determine  what  particular 
range  of  concentrations  it  was  desirable  to  work  with.  Judging  from  the 
color  of  the  solution  as  seen  by  the  unaided  eye  was  found  to  be  very 
unsatisfactory,  since  many  solutions  have  very  wide  absorption  bands 
which  may  give  the  solution  quite  a  decided  tint,  even  when  the  absorp- 
tion is  so  feeble  that  it  would  be  almost  impossible  to  obtain  a  satisfactory 
photographic  record  of  it.  On  the  other  hand,  when  solutions  have  nar- 
row, intense  absorption  bands,  like  the  salts  of  the  rare  earth  metals,  the 
solution  may  show  practically  no  color  to  the  unaided  eye,  and  still  the 
absorption  bands  may  be  quite  intense  as  seen  in  the  spectroscope. 

For  photographing  the  spectra,  the  vertical  grating  spectrograph  used 
by  Jones  and  Uhler  was  employed.  In  its  original  form  as  used  by  them 
films  2.5  by  7  inches  were  employed,  on  which  the  spectrum  from  X  2000 
to  X  6300  could  be  registered. 

In  the  present  work  it  was  decided  to  remodel  it  so  as  to  allow  the 
whole  spectrum  from  X  2000  to  X  7600  to  be  photographed,  and  accord- 
ingly the  camera  and  the  camera  end  of  the  box  were  enlarged  so  as  to 
hold  films  2.5  by  9  inches.  Owing  to  the  fact  that  the  grating  has  only 
10,000  lines  to  the  inch,  it  was  not  possible  to  add  the  extra  2  inches  to 
the  red  end  of  the  camera,  which,  if  it  could  have  been  done,  would  have 
left  the  grating-axis  very  near  the  middle  of  the  spectrum;  the  2  inches 
being  actually  added  to  the  ultra-violet  end,  which  necessitated  turning 
the  grating-axis  farther  away  from  the  slit,  thus  placing  it  in  a  point  of 
the  spectrum  some  little  distance  beyond  the  visible  violet.  The  spec- 
trum is,  however,  near  enough  to  normal,  even  at  the  extreme  red  end, 
to  make  any  correction  unnecessary,  unless  measurements  of  a  very  high 
degree  of  accuracy  are  required. 

For  holding  the  aqueous  solutions,  the  cell  illustrated  in  figure  66, 
page  172,  of  "Hydrates  in  Aqueous  Solution,"  was  employed  throughout, 
while  for  non-aqueous  solvents  the  cell  shown  by  Plate  22  of  the  same  work 
was  used. 


INTRODUCTORY.  7 

PHOTOGRAPHIC    MATERIAL,  ETC. 

It  was  at  first  decided  to  use  films  coated  with  the  panchromatic  emul- 
sion made  by  Wratten  and  Wainwright,  of  Croyden,  England,  as  this  emul- 
sion has  been  found  to  be  very  uniformly  sensitive  to  light  of  all  wave- 
lengths between  X  2000  and  about  X  7400.  The  makers,  however,  did  not 
succeed  in  producing  a  satisfactory  film  in  time  for  the  present  work,  and 
hence  it  was  necessary  to  use  the  Seed  L-Ortho-film  for  the  region  from 
X  2000  to  about  A  6000,  and  to  make  a  separate  exposure  for  the  red  end  of 
the  spectrum,  using  for  this  purpose  Wratten  and  Wainwright  panchro- 
matic glass  plates,  cut  to  such  lengths  (4  to  4.5  inches)  that  the  curvature 
of  the  focal  plane  would  not  introduce  any  appreciable  difficulty.  This 
method  very  nearly  doubled  the  time  and  work  consumed  in  making  the 
spectrograms,  as  two  separate  sets  of  exposures  had  to  be  made  with  each 
set  of  solutions.  It  also  made  it  very  difiicult  to  get  the  two  negatives  of 
such  intensities  that  they  would  match  satisfactorily,  owing  not  only  to 
the  different  absolute  sensibility  of  the  two  emulsions  to  light  of  a  given 
wave-length,  but  also  to  the  different  rates  at  which  the  photographic  black- 
ening increases  with  time  for  the  two  emulsions. 

Plate  1  gives  some  idea  of  the  sensibility  of  the  photographic  plates 
used  to  light  of  different  wave-lengths,  and  also  shows  how  the  photo- 
graphic action  increases  with  time  of  exposure.  A  is  a  series  of  exposures 
of  the  Wratten  plate  to  the  spectrum  of  the  Nernst  filament,  the  times  of 
exposure  being  2,  4,  6,  8,  15,  and  30  seconds,  and  1  and  2  minutes,  respec- 
tively. 0.8  ampere  of  alternating  current  flowed  through  the  filament 
and  the  slit  was  adjusted  to  a  width  of  0.01  cm.  The  strip  on  the  nega- 
tive corresponding  to  the  2-seconds  exposure  shows  some  photographic 
action  from  X  3800  to  X  7250  with  faint  maxima  near  X  4700,  X  5400,  X  5950, 
X  6500,  and  X  6950,  respectively;  the  corresponding  minima  falling  near 
X  5050,  X  5600,  X  6100,  and  X  6700.  The  minimum  at  X  5050  is  much  more 
pronounced  than  any  of  the  others,  but  it  is  interesting  to  note  that  even 
at  the  middle  of  this  one  there  is  considerable  photographic  blackening 
produced  by  the  2-seconds  exposure.  The  maxima  and  minima  show 
most  distinctly  in  the  two  strips  corresponding  to  6-seconds  and  8-second3 
exposure,  respectively;  which  shows  that  at  first  the  photographic  action 
in  the  maxima  increases  with  time  somewhat  more  rapidly  than  in  the 
minima.  The  X  5050  minimum  is  still  visible  in  the  1-minute  exposure, 
while  the  others  can  be  seen  only  with  difficulty.  In  a  full  exposure  (1  to 
2  minutes)  the  photographic  action  in  the  red  begins  to  shade  off  percep- 
tibly at  X  7250,  but  is  still  considerable  as  far  as  X  7500.  B  is  a  series  of 
exposures  of  the  Seed  L-Ortho-film;  the  successive  times  of  exposure 
being  2,  3,  4,  8,  15,  and  30  seconds,  and  1  minute,  respectively.  The  cur- 
rent in  the  filament,  width  of  slit,  and  development  were  exactly  the  same 
as  used  in  making  the  negative  for  A.  The  strip  corresponding  to  the  2- 
seconds  exposure  shows  maxima  at  X  4700  and  X  5600,  and  a  minimum  at 
X  5250,  at  the  center  of  which  the  negative  records  no  photographic  action 
whatever.  The  maximum  at  X  4700  in  the  2-seconds  exposure  is  slightly 
stronger  than  the  one  at  X  5600,  while  the  reverse  is  the  case  in  the  strips 
corresponding  to  exposure  of  4  seconds  or  more.     The  action  increases 


8  ABSORPTION     SPECTRA    OF    SOLUTIONS. 

with  time  much  more  rapidly  with  yellow  light  than  with  blue  light.  The 
action  in  the  minimum  also  increases  more  rapidly  with  time  than  is  the 
case  with  the  ^  5050  minimum  of  the  Wratten  plate.  With  a  full  exposure 
(1  minute)  blackening  of  the  Seed  film  begins  to  shade  off  at  X  5900,  but 
may  be  seen  as  far  as  X  6200. 

The  developer  used  throughout  was  a  concentrated  hydrochinone 
solution  made  up  according  to  Jewell's  formula  (Astrophys.  Journ.,  1900, 
pp.  240-243). 

SOURCES    OF    LIGHT. 

The  most  satisfactory  source  for  the  region  of  the  spectrum  lying 
between  the  extreme  red  and  the  beginning  of  the  ultra-violet  is  the  Nernst 
lamp,  as  it  is  brilhant  enough  to  bring  the  time  exposure  down  to  about  a 
minute,  and  is,  of  course,  perfectly  continuous  and  steady.  In  the  ultra- 
violet the  spectrogram  on  Plate  1  shows  that  its  action  decreases  rapidly 
with  the  wave-length,  ceasing  practically  at  about  X  3200.  For  this  region, 
then,  some  spark-spectrum  must  be  used.  The  cadmium  zinc  spark  used 
by  Jones  and  Uhler  is  fairly  satisfactory,  being  especially  strong  in  the 
extreme  ultra-violet,  but  it  has  the  disadvantage  of  having  a  limited 
number  of  very  intense  Hnes  on  a  rather  faint,  continuous  background. 
It  was  hoped  that  some  spark-spectrum  could  be  found  having  a  very  large 
number  of  lines,  but  without  any  lines  of  very  great  intensity. 

A  reference  to  published  tables  of  the  spectra  of  the  elements  showed 
that  tungsten,  molybdenum,  and  uranium  all  satisfied  this  requirement. 
Each  of  these  has  so  many  lines,  and  these  so  closely  packed,  that  with 
an  instrument  of  moderate  dispersion  the  spectrum  ought  to  be  nearly 
continuous.  The  problem  was  to  make  spark  terminals  of  these  sub- 
stances which  could  be  used  satisfactorily.  The  metals  not  being  easily 
obtainable,  the  following  plan  was  tried:  Sheet  carbon  about  3  mm.  thick 
was  cut  into  pieces  about  1  cm.  by  4  cm.  and  dipped  into  concentrated 
solutions  of  ammonium  molybdate,  or  uranium  nitrate;  a  suitable  solu- 
tion of  a  tungstate  was  not  tried.  These  pieces  of  carbon  were  then  heated 
to  redness  in  a  Bunsen  flame  and  again  quickly  immersed  in  the  solution, 
the  process  being  repeated  two  or  three  times.  Some  pieces  were  also 
treated  with  solutions  of  salts  of  iron,  copper,  and  cobalt,  and  others 
were  treated  with  two  or  more  of  the  solutions  in  succession.  The  spectra 
of  these  carbon  terminals  showed  the  lines  of  the  metals  with  which  they 
were  treated  almost  as  well  as  terminals  of  the  metals  themselves;  but  in 
the  case  of  iron,  copper,  or  cobalt  the  cyanogen  bands  were  also  present. 
With  molybdenum  and  uranium,  however,  the  cyanogen  bands  were 
absent,  the  merest  trace  of  the  X  3883  band  appearing.  The  uranium 
spectrum  is  almost  continuous  with  the  dispersion  employed,  but  its 
intensity  falls  off  very  rapidly  from  X  3000  towards  the  ultra-violet.  The 
molybdenum  spectrum,  although  not  so  nearly  continuous,  is  much  richer 
in  ultra-violet,  being  quite  strong  as  far  as  X  2300. 

Curiously  enough,  if  a  pair  of  carbon  terminals  is  treated  with  molyb- 
denum and  also  with  one  or  more  of  the  other  metals,  very  Httle  except 
the  molybdenum  spectrum  is  seen.  Uranium,  however,  seems  to  increase 
the  intensity  of  the  continuous  background  somewhat,  and  hence  the  termi- 


INTRODUCTORY.  9 

nals  finally  used  were  prepared  by  dipping  twice  in  the  molybdate  solution 
and  then  three  times  in  the  solution  of  uranium  nitrate. 

It  was  also  found  that  the  terminals  charged  with  various  metals  wore 
away  at  quite  different  rates  when  the  spark  was  passed.  Those  dipped 
into  a  solution  of  a  copper  salt  wore  away  at  the  rate  of  a  millimeter  or 
more  per  minute,  while  those  treated  with  molybdenum  could  be  used  for 
hours  without  appreciable  wear.  The  character  of  the  spectrum  given 
by  these  molybdenum-uranium  terminals  may  be  seen  from  any  of  the 
plates  reproduced  in  the  following  chapters,  which  do  not  show  complete 
absorption  in  the  ultra-violet. 

The  coil  used  to  produce  the  spark  was  a  large  Rontgen  X-ray  coil, 
through  the  primary  of  which  was  passed  an  alternating  current  of  from 
5  to  8  amperes,  60  cycles;  and  across  the  secondary  terminals  was  shunted 
a  capacity  of  about  0.011  microfarad.  The  spark  used  was  about  a  centi- 
meter in  length,  and  was  placed  about  15  cm.  above  the  sht,  the  direction 
in  which  the  spark  passed  being  perpendicular  to  the  length  of  the  slit. 
By  this  arrangement  the  grating  received  light  from  all  parts  of  the  spark 
at  the  same  time.  In  order  to  produce  a  uniform  photographic  strip  of 
the  proper  width,  it  was  necessary  to  keep  the  spark  terminals  moving  in 
a  direction  parallel  to  the  length  of  the  slit,  which  was  done  by  hand;  a 
suitable  stand  being  used.  Care  was  taken  to  move  the  spark-holder 
always  at  the  same  rate,  namely,  a  to-and-fro  motion  was  executed  in 
about  4  seconds,  which  insured  equality  in  times  of  exposure  for  the  dif- 
ferent strips  of  the  spectrograms.  The  intensity  of  the  spark  undoubt- 
edly varied  somewhat,  due  to  fluctuations  of  the  voltage  impressed  upon 
the  primary  terminals  of  the  coil;  but  this  was  found  to  be  so  small  that 
no  provision  was  made  for  regulating  it.  The  Nernst  filament  is,  however, 
so  sensitive  to  sHght  changes  in  voltage  that  a  variable  resistance  was 
placed  in  series  with  it;  by  regulating  which  the  current  was  always  kept 
at  0.8  ampere  during  an  exposure. 

MAKING   A   SPECTROGRAM. 

In  making  a  spectrogram  consisting  of  seven  photographic  strips  with 
a  comparison  spectrum,  the  following  was  the  usual  sequence  of  opera- 
tions: Seven  separate  solutions  were  made  up,  the  quantity  of  each  being 
usually  25  c.c.  The  cell  to  be  used,  having  been  cleaned  and  dried,  was 
filled  to  the  required  depth  with  the  most  concentrated  solution  of  the 
series,  and  the  quartz  plates  limiting  the  depth  of  the  solution  adjusted  to 
parallelism.  The  exposure  to  the  Nernst  lamp  was  then  made,  the  cur- 
rent being  kept  at  0.8  ampere  by  hand  regulation  of  the  variable  resist- 
ance in  series  with  it.  The  usual  time  of  this  exposure  was  1  minute.  An 
opaque  screen  covering  up  the  visible  spectrum  as  far  down  as  X  4000  was 
then  interposed  between  the  grating  and  the  photographic  film,  and  the 
exposure  to  the  light  of  the  spark  in  the  ultra-violet  was  made.  The 
duration  of  this  exposure  was  usually  2  minutes.  The  photographic  film 
was  then  moved  a  distance  of  6.5  mm.  into  the  proper  position  for  the 
next  exposure.  The  cell  was  emptied  and  rinsed  out  with  a  few  drops  of 
the  next  solution,  and  the  series  of  operations  repeated  for  the  second 


10  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

strip,  and  so  on.  It  was  always  found  advisable  to  clean  the  slit-jaws  after 
each  exposure,  and  also  to  see  that  the  image  of  the  Nernst  filament  fell  in 
the  proper  position  on  the  slit. 

After  the  film  had  been  exposed  and  the  comparison  spark  spectrum 
impressed,  it  was  necessary  to  make  a  series  of  exposures  on  a  panchro- 
matic plate  for  the  red  end  of  the  spectrum,  using  the  same  set  of  solutions. 
No  exposure  to  the  spark  was  made  in  this  set  except  for  the  narrow  com- 
parison strip.  As  the  extreme  red  end  of  the  plate  was  at  about  X  7600, 
A  3800  of  the  second  order  would  overlap  the  first  order  here.  Accordingly 
an  absorbing  screen  was  always  used  in  making  the  exposures  for  the  red 
end  of  the  spectrum.  This  screen  consisted  of  two  glass  plates  separated 
by  a  layer  of  Canada  balsam  a  little  less  than  a  millimeter  thick.  It 
absorbed  all  radiations  of  shorter  wave-length  than  }.  3900. 

The  scale  accompanying  the  spectrograms  in  the  following  chapters 
was  made  by  photographing  an  ordinary  paper  scale.  Several  photo- 
graphs were  made,  the  distance  between  the  paper  scale  and  the  lens  of 
the  camera  being  varied  slightly  from  exposure  to  exposure.  The  resulting 
negative  which  fitted  the  majority  of  the  spectrograms  best  was  selected 
and  used  throughout.  Absolute  accuracy  is  not  to  be  expected,  owing  to 
the  fact  that  both  photographic  films  and  the  paper  on  which  prints  from 
these  were  made,  contract  more  or  less  in  drying,  and  different  films  or 
papers  contract  differently,  k  5000  on  the  scale  was  always  placed  in  coin- 
cidence with  the  corresponding  wave-length  on  the  photographic  strips; 
the  correction  for  the  ends  of  the  spectrograms  differs  slightly  for  the 
different  plates,  but  never  amounts  to  more  than  about  25  or  30  Ang- 
strom units. 


CHAPTER  II. 

SALTS  OF  COBALT. 

Jones  and  Uhler/  in  their  work  on  the  absorption  spectra  of  salts  of 
cobalt  and  copper,  discussed  a  number  of  the  more  important  papers 
dealing  with  cobalt,  so  that  it  is  necessary  only  to  make  brief  reference 
to  them  here. 

Babo  -  observed  a  number  of  the  color  changes  produced  in  cobalt 
salts  by  change  in  temperature,  or  by  addition  of  a  dehydrating  agent. 
Similar  observations  were  made  by  Gladstone  ^  and  Schiff.* 

Bersch^  proposed  the  view  that  there  are  two  modifications  of  the 
compound  C0CI2.6H2O — the  one  red  and  the  other  blue — and  would  thus 
account  for  the  color  changes  in  cobalt  salts  with  rise  in  temperature. 

Tichborne '  would  also  account  for  these  color  changes  on  the  basis  of 
hydration,  and  Vogel's  ^  work  pointed  in  the  same  direction. 

The  investigations  of  Russell*  on  the  absorption  spectra  of  solutions  of 
cobalt  salts  are  important.  He  worked  under  various  conditions,  such  as 
with  the  fused  salt,  with  its  solution  in  concentrated  hydrochloric  acid, 
and  with  solutions  in  the  various  alcohols  and  glycerol.  He  also  studied 
the  effect  of  changes  in  temperature  on  the  absorption  spectra.  He  con- 
cluded, as  the  result  of  all  of  his  work,  that  the  color  of  the  aqueous  solu- 
tions was  due  to  the  presence  of  hydrates. 

Potilitzin  ^  showed  that  the  conclusion  of  Bersch,  that  there  are  two 
modifications  of  the  compound  CoClj.GHjO,  is  an  error,  and  that  the  for- 
mation of  blue  cobalt  chloride  from  the  red  modification  is  always  a  dehy- 
dration phenomenon. 

Etard  ^^  studied  the  color  changes  and  also  the  solubility  curve  of 
cobalt  chloride  and  iodide,  and  showed  from  the  sudden  changes  in  the 
direction  of  the  solubility  curves  the  existence  of  various  hydrates  of  these 
salts.  He  also  studied  the  changes  in  the  absorption  spectra  of  cobalt 
chloride  with  changes  in  temperature. 

Engel "  does  not  believe  that  any  general  theory  can  be  advanced  to 
account  for  the  changes  in  color  of  cobalt  salts,  but  thinks  that  the  blue 
color  is  often  due  to  the  formation  of  double  compounds.  The  blue  color 
of  a  hot,  saturated  solution  of  cobalt  chloride  he  regards  as  due  to  a  double 

*  Publication  No.  60,  Carnegie  Institution  of  Washington. 
»  Jahresber.,  1857,  72. 

•Journ.  Chem.  Soc,  10,  79  (1859). 
«  Lieb.  Ann.,  110,  203  (1859). 

•  Sitzungsber.  Wien  Akad.,  11,  56,  726  (1867). 
•Chem.  News,  25,  133  (1872). 

» Ber.  d.  deutsch.  chem.  Gesell.,  8,  1533  (1875);  11,  913  (1878);  12,  2313  (1879). 

•Proc.  Roy.  Soc,  32,  258  (1881).     Chem.  News,  59,  93  (1889). 

•Ber.  d.  deutsch.  chem.   Gesell.,  17,  276    (1884),  and  Bull.  Soc.  Chim.  (3),  6,  264 

(1891). 
loCompt.  rend.,  120,  1057  (1895);  131,  699  (1900). 
"BuU.  Soc.  Chim.  (3),  6,  239  (1891). 

11 


12  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

compound  between  the  salt  and  the  hydrochloric  acid  liberated  as  the 
result  of  hydrolysis. 

Wyroubofif  ^  and  Le  Chatelier  ^  show  that  Engel's  view  is  untenable. 

W.  N.  Hartley/  in  his  elaborate  investigations  on  absorption  spectra, 
has  included  a  number  of  salts  of  cobalt.  His  experimental  work  con- 
sisted in  observing  and  photographing  the  spectra  of  a  large  number  of 
solutions  of  chlorides,  bromides,  iodides,  nitrates,  etc.,  of  a  fairly  large 
number  of  metals,  including  cobalt.  Some  of  the  more  interesting  and 
important  conclusions  at  which  he  arrived  are  the  following,  stated  nearly 
in  his  own  words. 

When  a  definite  crystalline  hydrate  is  dissolved  in  a  non-aqueous  sol- 
vent, upon  which  it  does  not  act  chemically,  the  molecules  of  the  salt  re- 
main unchanged  in  chemical  composition. 

In  a  series  of  anhydrous  salts  which  do  not  form  definite  crystalline 
hydrates,  the  efifect  of  rise  in  temperature  up  to  100°  C.  does  not  produce 
any  alteration  in  their  absorption  spectra,  other  than  that  which  results 
with  substances  which  undergo  no  chemical  change  with  such  rise  in 
temperature.  The  change  in  question  is  usually  an  increase  in  the  inten- 
sity of  the  absorption,  or  a  slight  widening  of  the  absorption  bands. 

Crystallized  hydrated  salts  dissolved  in  a  minimum  amount  of  water  at 
20°  C.  undergo  dissociation  by  rise  in  temperature.  The  extent  of  the  dis- 
sociation may  proceed  as  far  as  complete  dehydration  of  the  compounds, 
so  that  more  or  less  of  the  anhydrous  salt  may  be  formed  in  the  solution. 

The  most  stable  compound  that  can  exist  in  a  saturated  solution  at 
16°  C.  or  20°  C.  is  not  always  of  the  same  composition  as  the  molecule  of 
the  crystallized  solid  at  the  same  temperature,  since  the  solid  may  undergo 
a  partial  dissociation  from  its  water  of  crystallization  when  the  molecule 
enters  into  solution.  When  a  saturated  solution  of  a  colored  salt  under- 
goes a  great  change  in  color  or  any  remarkable  change  in  its  absorption 
spectrum  upon  dilution,  the  dilution  is  always  accompanied  by  marked 
heat  evolution. 

Hartley*  at  the  close  of  his  paper  on  "The  Absorption  Spectra  of 
Metallic  Nitrates"  has  the  following  significant  paragraph. 

The  ultimate  conclusion  drawn  from  this  work  is  that  the  operations  of  dissolving  a 
salt  and  diluting  the  solution  do  not  cause  a  separation  of  the  compound  into  ions,  but 
only  a  dissociation  of  such  a  character  that  the  molecule  is  shown  to  consist  of  two  parts, 
the  movements  of  the  one  being  influenced  by  those  of  the  other,  so  that  the  molecule  of 
the  salt  is,  in  fact,  not  completely  resolved  into  ions,  but  is  in  a  condition  of  molecular 
tension.  The  application  of  external  energy,  such  as  light  or  electricity,  may,  however, 
readily  cause  a  separation  such  as  may  be  brought  about  by  electrolysis  or  by  static  elec- 
tricity, and  in  some  instances,  by  photographic  action. 

Ostwald  ^  thinks  that  the  red  color  of  solutions  of  salts  of  cobalt  is  due 
to  the  cobalt  ions. 

iBull.  Soc.  Chim.  (3)  6,  3  (1891). 

'Ibid.  (3),  6,  84  (1891). 

3  Dublin  Trans.  (2),  7,  253-312  (1900),  and  Joum.  Chem.  Soc,  81,  571  (1902);  83, 

221  (1903). 
*  Joum.  Chem.  Soc,  83,  245  (1903). 
'  Grundlinien  d.  anorg.  Chem.,  620. 


SALTS    OF    COBALT.  13 

The  paper  by  Donnan  and  Bassett  *  should  be  especially  mentioned  in 
connection  with  the  changes  in  color  of  cobalt  salts.  After  citing  a  num- 
ber of  well-known  facts,  and  adding  a  fairly  large  number  of  interesting 
new  ones,  they  came  to  the  conclusion  that  the  blue  color  of  solutions  of 
cobalt  salts  is  due  to  the  formation  of  complex  anions  containing  cobalt. 
Some  of  the  evidence  which  they  furnish  merits  very  careful  consideration 
in  this  connection. 

Hartley^  takes  issue  with  the  conclusions  reached  by  Donnan  and 
Bassett,  interpreting  the  facts  cited  or  discovered  by  them  in  terms  of 
hydration  and  dehydration. 

Cobalt  Chloride  in  Water — Beer's  Law.     (See  Plate  2  A  and  B.) 

In  both  A  and  B,  the  strip  corresponding  to  the  most  concentrated 
solution  is  adjacent  to  the  numbered  scale.  The  concentrations  of  the 
solutions  used  in  making  set  A  were  2.5,  1.88,  1.25,  0.83,  0.58,  0.42,  and  0.31, 
respectively;  the  corresponding  depths  of  cell  were  3,  4,  6,  9,  13,  18,  and  24 
mm.  The  concentrations  used  in  making  set  B  were  0.83,  0.63,  0.42, 
0.276,  0.192,  0.139,  and  0.104;  the  depths  of  cell  were  the  same  as  in  set 
A.  The  exposures  to  the  red  end  of  the  spectrum  were  omitted  in  this 
case,  inasmuch  as  observations  with  the  direct-vision  spectroscope  showed 
that  the  solutions,  at  least  in  such  thicknesses  of  layer  as  were  employed, 
were  perfectly  transparent  from  the  beginning  of  the  orange  to  the  end  of 
the  red.  The  most  concentrated  solution  in  layers  of  2  cm.  or  more  showed 
faint  traces  of  bands  in  the  orange  and  red,  but  in  layers  of  a  few  milli- 
meters thickness  these  were  of  course  quite  invisible. 

The  spectrogram  shows  three  regions  of  absorption:  One  in  the  green, 
middle  near  A  5200;  one  in  the  ultra-violet,  with  its  middle  near  X  3300; 
and  one  in  the  extreme  ultra-violet.  The  strips  corresponding  to  the  four 
most  concentrated  solutions  of  set  A  show  only  one  absorption  band  in 
the  ultra-violet,  but  the  strip  corresponding  to  the  fifth  solution  shows 
transmission  between  A  2800  and  X  3000,  and  absorption  from  X  3000  to 
X  3500;  thus  making  it  very  evident  that  there  are  two  regions  of  absorp- 
tion. The  strips  corresponding  to  the  three  most  concentrated  solutions 
of  set  B  also  show  very  plainly  the  existence  of  the  band  at  X  3300,  al- 
though the  absorption  is  not  complete  even  at  the  middle  of  the  band. 
In  the  fourth  strip  of  B,  corresponding  to  a  concentration  of  0.276  and 
depth  of  cell  equal  to  9  mm.,  practically  all  trace  of  the  band  has  disap- 
peared, the  spark  spectrum  appearing  to  shade  off  uniformly  from  X  3600 
to  X  2650,  where  it  ends. 

It  will  be  noticed  that  the  intensity  of  the  spark  spectrum  in  the 
region  X  2900  is  greater  for  the  strips  near  the  numbered  scale  than  for 
the  fourth,  fifth,  and  sixth  strips.  This  is  due  partly  to  a  gradual  de- 
crease in  the  intensity  of  the  spark  itself,  while  the  spectrogram  was  made, 
produced  by  a  gradual  fall  in  potential  of  the  source  of  alternating  cur- 
rent operating  the  coil.  That  the  effect  is  real,  however,  is  shown  by 
Plate  3  B,  where  a  similar  decrease  in  transmission  at  X  2900  with  dilu- 

>  Journ.  Chem.  See,  81,  939  (1902).  » Ibid.,  83,  401  (1903). 


14  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

tion  is  recorded,  and  where  the  intensity  of  the  spark  was  constant  or 
very  nearly  so  throughout.  The  increased  intensity  of  the  seventh  strip 
of  Plate  2  B  is  due  to  the  fact  that  before  making  the  exposure  the  poten- 
tial was  readjusted  to  its  original  value. 

The  absorption  in  the  extreme  ultra-violet  decreases  regularly  with 
dilution,  the  strip  corresponding  to  the  most  concentrated  solution  in  set  B 
transmitting  as  far  as  X  2650,  while  that  corresponding  to  the  most  dilute 
solution  shows  transmission  a  little  below  X  2500. 

In  set  A  the  band  in  the  green  narrows  quite  rapidly  at  first,  then 
more  slowly  with  dilution.  In  set  B  it  remains  very  nearly  constant, 
especially  in  the  strips  corresponding  to  the  more  dilute  solutions. 

The  facts  brought  out  by  the  spectrogram  may  be  summed  up  briefly 
as  follows:  When  the  product  of  concentration  and  depth  of  absorbing 
layer  is  kept  constant,  the  absorption  band  in  the  green  remains  constant, 
except  in  very  concentrated  solutions,  where  it  narrows  somewhat  with 
dilution;  the  one-sided,  extreme,  ultra-violet  absorption  decreases  with 
dilution,  while  the  band  having  its  center  near  X  3300  narrows  very  rapidly 
with  dilution,  disappearing  entirely  when  a  concentration  of  about  quar- 
ter-normal is  used  in  a  layer  about  1  cm.  in  depth.  In  the  region  X  2800 
to  X  3000  there  is  remarkable  transparency  in  solutions  having  a  concen- 
tration about  half-normal.  This  transparency  decreases  somewhat  with 
dilution  to  about  one-tenth  normal. 

A  comparison  between  this  plate  and  Plate  2  in  the  work  of  Jones  and 
Uhler  already  referred  to,  reveals  a  remarkable  difference  in  the  ultra- 
violet. The  four  strips  nearest  the  comparison  spectrum  of  their  plate 
correspond  to  concentrations  of  more  than  2.0  normal,  and  with  a  depth 
of  absorbing  layer  of  0.67  cm.,  and  yet  the  absorption  in  the  region 
X  3000  to  X  3500  is  not  very  great.  It  might  at  first  sight  seem  probable  that 
the  absorption  band  in  this  region  was  produced  by  some  impurity  in  the 
cobalt  chloride  used.  But  this  is  improbable,  since  the  salt  from  which 
were  obtained  the  solutions  used  by  Jones  and  Uhler  and  those  used  in 
the  present  work  came  from  the  same  sample  of  material.  Besides,  Plate 
23,  in  "Hydrates  in  Aqueous  Solution,"  shows  that  the  solution  of  the 
cobalt  chloride  used  by  Jones  and  Uhler,  when  dissolved  in  methyl  alco- 
hol, exerts  strong  absorption  in  the  ultra-violet,  while  Plate  7  of  the  pres- 
ent work  indicates  only  faint  absorption  in  this  region,  the  concentrations, 
depth  of  cell,  etc.,  used  in  the  two  cases,  not  differing  materially;  hence,  it 
is  evident  that  it  is  not  a  question  of  the  presence  of  an  impurity.  The 
probable  explanation  is  that  the  discrepancy  is  due  to  a  difference  of  tem- 
perature. 

It  is  well  known  that  the  absorption  of  cobalt  salts  in  solution  is 
greatly  affected  by  even  slight  changes  in  temperature,  and  if  the  X  3300 
band  is  especially  sensitive,  such  variations  of  temperature  as  occur  in 
the  laboratory  on  different  days  might  be  sufiicient  to  account  for  the 
changes  in  the  spectrum  observed.  No  data  are  at  hand,  however,  giving 
the  change  of  absorption  in  the  ultra-violet  produced  by  change  of  tempera- 
ture, and  hence  this  point  will  have  to  remain  unsettled  until  the  work  now 
in  progress  shall  have  supplied  the  data  for  the  absorption  band  in  question. 


SALTS    OF   COBALT.  15 

Cobalt  Chloride  in  Water — Number  of  Ions  in  the  Path  of  the  Beam 
OF  Light  Constant.     (See  Plate  39  B.) 

The  concentrations  were  2.00,  0.99,  0.546,  0.318,  0.205,  0.140,  and 
0.103,  the  corresponding  depths  of  absorbing  layer  being  3,  4,  6,  9,  13, 
18,  and  24  mm.,  respectively. 

Very  little  need  be  said  about  this  spectrogram,  since  it  shows  that 
the  absorption  decreases  rapidly  with  dilution,  as  was  to  be  expected  from 
the  result  when  the  product  of  concentration  and  depth  of  absorbing  layer 
was  kept  constant.  This  spectrogram  also  shows  the  existence  of  the  band 
at  yl  3300,  as  well  as  indications  of  the  decrease  of  transparency  at  X  2900 
with  dilution  from  half-normal  to  tenth-normal.  That  an  increase  in 
absorption  in  this  region  is  indicated  in  this  spectrogram  is  significant, 
inasmuch  as  it  shows  that  dissociation  is  not  able  to  account  for  it.  For 
the  reason  stated  in  the  description  of  Plate  2,  no  exposure  was  made 
to  the  red  end  of  the  spectrum  for  this  set  of  solutions. 

Cobalt  Chloride  in  Water — Molecules  Constant.    (See  Plate  3.) 

In  both  sets  the  strip  corresponding  to  the  most  concentrated  solution 
is  adjacent  to  the  numbered  scale.  The  solutions  were  made  up  of  such 
concentrations  that  the  number  of  undissociated  molecules  of  cobalt 
chloride  in  the  path  of  the  light  should  be  constant  for  each  set.  The 
concentrations  for  set  A  were  2.50,  2.04,  1.53,  1.14,  0.885,  0.695,  and 
0.570;  the  corresponding  depths  of  absorbing  layer  were  3,  4,  6,  9,  13, 
18,  and  24  mm.,  respectively.  The  concentrations  for  set  B  were  0.70, 
0.575,  0.434,  0.329,  0.253,  0.197,  0.158.  The  data  used  in  calculating  the 
concentrations  were  taken  from  the  table  on  page  81  of  "Hydrates  in 
Aqueous  Solution." 

In  making  the  exposures  for  set  A  only  the  light  from  the  Nernst 
lamp  was  used,  as  a  preliminary  test  showed  that  none  of  the  solutions 
transmitted  any  light  of  shorter  wave-length  than  X  3850.  Owing  to  the 
general  absorption  of  concentrated  solutions  of  cobalt  chloride,  it  was 
necessary  to  increase  the  time  of  exposure,  and  hence  for  set  A  the  light 
from  the  Nernst  lamp  was  allowed  to  act  for  a  period  of  3  minutes  through 
each  of  the  solutions. 

The  exposures  for  set  B  were  1^  and  3  minutes,  respectively,  for  the 
Nernst  lamp  and  the  spark.  No  exposures  were  made  to  the  red  end  of 
the  spectrum,  since  all  the  solutions  showed  uniform  transmission  to  be- 
yond >l  7600. 

A  shows  that  the  band  having  its  center  at  X  3300  narrows  with  dilu- 
tion, even  when  the  number  of  molecules  in  the  path  of  the  beam  of  light 
is  kept  constant;  thus  demonstrating  that  the  change  in  this  band  can 
not  be  accounted  for  by  dissociation.  The  limits  of  transmission  as  given 
by  the  negative  for  the  seven  solutions  beginning  with  the  most  concen- 
trated are  X  4130,  X  4070,  X  3970,  X  3930,  X  3900,  X  3870,  X  3850,  which 
shows  that  the  band  narrows  more  rapidly  at  first.  In  B  this  band  may  be 
seen  in  the  three  strips  nearest  to  the  numbered  scale,  corresponding  to 
concentrations  0.70,  0.575,  and  0.434,  but  it  has  practically  disappeared 
in  the  fourth  strip,  corresponding  to  concentration  0.329.    The  maximum 


16  ABSORPTION    SPECTRA    OP    SOLUTIONS. 

of  transmission  near  X  2900  is  very  well  shown  in  B.  The  absorption  in 
the  extreme  ultra-violet  seems  to  remain  sensibly  constant  in  B,  the  limit 
of  transmission  being  }.  2700  throughout. 

The  band  in  the  green  is  peculiar.  In  A  its  violet  edge  moves  farther 
down  into  the  violet,  and  in  such  a  way  that  the  limits  of  transmission 
for  the  seven  strips  lie  almost  exactly  on  a  straight  line,  the  total  dis- 
placement being  100  Angstrom  units  or  from  k  4550  to  X  4450.  The  red 
edge  of  the  band  at  first  moves  towards  the  violet,  then  turns  and  moves 
towards  the  red  as  the  dilution  increases.  On  the  whole  the  absorption 
band  widens  with  dilution,  but  from  concentration  2.5  to  1.14  its  center 
moves  towards  the  violet  by  about  50  A.  U.,  after  which  it  remains  prac- 
tically unchanged  in  position.  In  B  the  band  widens  with  dilution  and 
very  nearly  uniformly,  the  apparent  asymmetry  being  due  to  the  greater 
sensitiveness  of  the  Seed  film  to  yellow  light. 

CoBAi/r  Chlobide  in  Methyl  AlcohoI/ — Beer's  Law.     (See  Plate  4.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A  were  0.25,  0.21,  0.174,  0.143,  0.118,  0.100,  and  0.083;  the  correspond- 
ing depths  of  cell  were  8,  9.5,  11.5,  14,  17,  20,  and  24  mm.,  respectively. 
For  B  the  concentrations  were  0.143,  0.120,  0.100,  0.082,  0.0675,  0.0572, 
and  0.0476;  the  depths  of  cell  were  the  same  as  for  set  A.  The  most  con- 
centrated solutions  had  a  very  deep  purple  color  which,  as  dilution  in- 
creased, changed  to  pink.  The  solutions  for  set  A  exerted  considerable 
general  absorption,  so  that  in  order  to  get  a  satisfactory  spectrogram  the 
slit  was  opened  to  0.015  cm.  and  the  exposures  to  the  Nernst  lamp  and 
spark  were,  respectively,  3  and  4^  minutes.  In  making  the  negative  for 
set  B  the  slit  was  set  at  0.01  cm.  and  the  exposures  were  IJ  and  3  minutes, 
respectively,  for  the  Nernst  lamp  and  spark. 

Both  A  and  B  show  a  region  of  absorption  in  the  extreme  ultra-violet 
which  narrows  slightly  with  dilution.  In  A  the  limits  of  transmission  for  the 
most  concentrated  and  most  dilute  solutions  are  X  2630  and  X  2600,  respec- 
tively, while  in  B  the  limit  is  at  X  2570  throughout.  The  strips  corresponding 
to  the  more  concentrated  solutions  (those  nearest  the  numbered  scale)  in  A 
show  the  presence  of  two  faint  absorption  bands,  having  centers  at  A  3100 
and  X  3600,  respectively.     Both  of  these  disappear  gradually  with  dilution. 

The  absorption  band  in  the  green  has  its  center  at  X  5250  in  both  A 
and  B.  It  narrows  somewhat  with  dilution  in  A,  whereas  in  B  it  remains 
of  sensibly  constant  width. 

In  the  orange  and  red  is  a  rather  complicated  group  of  absorption 
bands,  the  absorption  of  the  most  concentrated  solution  of  set  A  beginning 
at  about  X  5850  and  extending  to  a  little  beyond  X  7000.  With  dilution 
this  absorption  decreases  rapidly,  breaking  up  into  bands,  the  position 
and  character  of  which  are  as  follows: 

1.  Fairly  narrow  band,  with  center  at  X  5910;  appearing  on  the  nega- 
tive as  far  as  the  sixth  strip  in  A. 

2.  Band  with  center  at  X  6050,  which  is  visible  in  the  strip  corresponding 
to  the  fifth  solution  of  A.  This  band  and  the  one  at  X  5910  are  not  clearly 
separated  in  the  strips  corresponding  to  the  first  and  second  solutions. 


SALTS    OF    COBALT.  17 

3.  Fairly  narrow,  intense  band,  with  center  at  k  6240,  which  may  be 
seen  as  far  as  the  strip  corresponding  to  the  seventh  solution. 

4.  Faint,  rather  wide  band,  with  center  at  approximately  /i  6450,  and 
disappearing  practically  in  the  strip  corresponding  to  the  fourth  solution. 
This  band  is  nowhere  clearly  separated  from  the  larger  one  at  X  6700. 

5.  Intense,  wide  band,  with  center  at  k  6700,  which  narrows  rapidly 
with  dilution,  but  may  be  seen  quite  plainly  in  the  negative  strip  corre- 
sponding to  the  most  dilute  solution  of  set  A.  It  is  also  clearly  visible 
in  the  strips  corresponding  to  the  three  most  concentrated  solutions  of 
set  B,  while  the  other  bands  are  seen  only  with  difficulty  on  this  negative. 

The  transmission  from  X  7000  to  the  end  of  the  visible  red  is  complete 
for  all  the  solutions  referred  to  on  page  245  of  "Hydrates  in  Aqueous 
Solution."  In  describing  the  absorption  of  cobalt  chloride  in  methyl 
alcohol,  Jones  and  Uhler  say  with  reference  to  their  observations  on  the 
absorption  in  the  red :  "  If  the  solution  could  have  been  made  more  con- 
centrated, or  better,  if  the  cell  had  been  deeper,  it  is  extremely  probable 
that  all  the  bands  observed  in  aqueous  solutions  could  have  been  seen 
with  the  alcoholic  solution  in  question." 

The  above  description  of  the  bands  shown  on  Plate  4,  in  conjunction 
with  Jones  and  Uhler's  curves  on  page  197,  and  their  description  of  the 
absorption  of  cobalt  chloride  in  water  given  on  page  189,  will  show  how 
very  different  the  absorption  spectra  in  question  are.  Not  only  are  the 
locations  of  the  centers  of  the  bands  very  dififerent,  but  the  character  of 
the  group  is  so  changed  that  it  is  difficult  to  see  how  it  can  be  regarded  as 
the  same  set  of  bands.  Furthermore,  Jones  and  Uhler's  description  on 
page  189,  and  their  curves  on  page  197,  show  that  the  absorption  in  the 
concentrated  cobalt  chloride  solution  is  quite  different  from  that  ob- 
served when  a  large  amount  of  calcium  chloride  is  added  to  a  dilute  solu- 
tion of  the  salt.  In  the  first  case  the  three  absorption  bands  noted  had 
their  centers  at  X  7140,  X  6760,  and  X  6360,  whereas  the  curves  for  the  sec- 
ond case  show  that  these  figures  correspond  to  regions  of  maximum  trans- 
mission. It  might  be  argued  that  this  indicates  a  shift  of  the  position  of 
the  bands  with  addition  of  calcium  chloride,  but  this  is  answered  by  Jones 
and  Uhler,  who  find  that  with  the  addition  of  more  of  this  salt  the  bands 
simply  increase  in  intensity  without  change  of  position.  The  shift  may, 
however,  be  due  to  a  change  in  the  concentration  of  the  cobalt  salt,  but 
this  point  has  yet  to  be  investigated. 

Cobalt  Chloride  in  Ethyl  Alcohol — Beer's  Law.     (See  Plate  6.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A 
were  0.15,  0.126,  0.104,  0.086,  0.071,  0.060,  and  0.050;  the  corresponding 
depths  of  cell  were  8,  9.5,  11.5,  14,  17,  20,  and  24  mm.,  respectively.  For  B 
the  concentrations  were  0.060,  0.050,  0.042,  0.034,  0.028,  0.024,  and  0.020; 
the  corresponding  depths  of  cell  being  the  same  as  used  in  making  set  A. 

The  color  of  the  solutions  as  seen  in  the  bottles  was  deep  blue  for  the 

most  concentrated,  changing  to  a  hght  greenish-blue  in  the  most  dilute. 

The  general  absorption  observed  in  the  concentrated  solutions  in  methyl 

alcohol  was  quite  absent  in  ethyl  alcohol,  so  that  in  making  the  negatives 

2 


18  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

for  both  A  and  B  the  sHt  was  adjusted  to  a  width  of  0.01  cm.  and  the  expo- 
sures to  the  Nernst  lamp  and  spark  were  Ih  and  3  minutes,  respectively. 

The  spectrogram  shows  a  region  of  absorption  in  the  extreme  ultra- 
violet, which  narrows  regularly  with  dilution;  the  limits  of  transmission 
for  the  strips  corresponding  to  the  most  concentrated  and  most  dilute 
solutions  of  set  A  being,  respectively,  X  2670,  and  A  2630,  while  for  set  B 
they  are  X  2580  and  X  2550. 

The  negative  for  set  A  also  shows  two  absorption  bands  in  the  ultra- 
violet, having  their  centers  roughly  at  X  3100  and  X  3600.  These  bands 
are  undoubtedly  identical  with  those  already  noted  in  the  same  region 
for  the  solutions  in  methyl  alcohol.  One  difference,  however,  is  to  be 
pointed  out.  The  bands  in  the  ethyl  alcohol  solutions  are  somewhat  more 
intense,  in  spite  of  the  fact  that  the  concentration  of  the  colored  salt  is 
smaller,  and  they  also  disappear  much  more  slowly  with  dilution. 

The  absorption  in  the  green  is  very  much  fainter  in  ethyl  alcohol  than 
in  methyl  alcohol;  and  measurement  of  the  position  of  the  center  of  the 
band  indicates  that  it  is  located  a  trifle  nearer  the  red  in  the  former  case. 
The  position  of  the  center  was  found  to  be  at  X  5280.  The  band  remains 
constant  in  width  and  position  with  dilution;  A  shows  a  region  of  intense 
absorption  in  the  yellow,  orange,  and  red,  which  narrows  somewhat  with 
decrease  in  concentration;  the  limits  of  transmission  for  the  most  concen- 
trated solution  being  X  5500  and  X  7170,  and  for  the  most  dilute  X  5500 
and  X  7060. 

In  B  the  absorption  in  this  region  of  the  spectrum  is  less  intense,  espe- 
cially for  the  more  dilute  solutions,  which  show  faint  transmission  through- 
out the  band.  The  red  edge  of  the  band  for  the  most  concentrated  solution 
is  at  X  6970,  while  for  the  most  dilute  solution  it  is  at  X  6800,  the  wave- 
lengths given  corresponding  to  the  limits  of  transmission  as  indicated  by 
the  photographic  blackening  of  the  negative.  The  strips  corresponding  to 
the  more  dilute  solutions  of  set  B  show  faint  traces  of  bands,  which  appear 
to  correspond  roughly  in  position  to  the  bands  noted  in  the  description  of 
Plate  4.  The  bands  at  X  5910  and  X  6050  only  show  faintly,  and  they  both 
appear  much  less  clearly  defined  than  in  the  methyl  alcohol  solutions. 

Cobalt  Chloride  in  Acetone — Beer's  Law.    (See  Plate  6.) 

The  concentrations  of  the  solutions  used  in  the  negative  for  A  were 
0.0154,  0.0129,  0.0107,  0.0088,  0.0073,  0.0060,  and  0.0051;  the  corre- 
sponding depths  of  cell  were  8,  9.5,  11.5,  14,  17,  20,  and  24  mm.,  respec- 
tively. For  set  B  the  concentrations  were  0.0073,  0.0061,  0.0051,  0.0042, 
0.0034,  0.0029,  and  0.0024;  the  depths  of  cell  were  the  same  as  in  A.  The 
solutions  were  all  blue,  the  color  merely  changing  from  a  dark  to  a  rather 
light  blue  with  dilution. 

The  spectrogram  shows  two  regions  of  absorption,  one  in  the  ultra- 
violet and  one  in  the  red.  The  faint  absorption  indicated  in  the  green 
is  due  to  the  diminished  sensibility  of  the  Seed  film  in  this  region. 

The  absorption  in  the  ultra-violet  is  due  entirely  to  the  solvent,  which 
may  be  inferred  from  the  fact  that  it  is  the  same  in  A  and  B,  and  also 
from  the  fact  that  it  increases  with  dilution.    This,  of  course,  is  due  to  the 


SALTS    OF    COBALT.  19 

increased  depth  of  the  layer  of  the  solvent.  The  limit  of  transmission  for 
the  8-mm.  layer  falls  at  X  3260,  and  for  the  24-mm.  layer  at  about  k  3320. 
The  absorption  in  the  red  is  seen  to  be  unchanged  by  dilution,  or,  as 
it  may  be  stated,  it  obeys  perfectly  Beer's  law.  In  A  it  consists  of  a  band 
extending  from  X  5500  to  }.  7100,  with  faint  transmission  near  X  6000; 
thus  indicating  a  band  of  absorption  at  X  5700.  B  shows  three  wide  and 
poorly-defined  absorption  bands,  having  their  centers  at  X  5725,  X  6200, 
and  X  6780,  respectively.  Of  these,  the  last  is  much  more  intense  than  any 
of  the  others.  No  breaking  up  of  these  bands  into  narrower  ones,  as  ob- 
served in  solutions  in  the  alcohols,  can  be  noticed,  although  the  trans- 
mission throughout  the  region  of  absorption  in  B  is  sujS5cient  to  allow  the 
narrower  bands  to  be  seen  if  they  existed.  We  must  conclude,  therefore, 
that  the  absorption  spectrum  of  the  acetone  solution  differs  considerably 
from  that  of  the  alcohol  solutions,  which  resemble  each  other  very  closely. 

Cobalt  Chloride  in  Methyl  Alcohol  with  Water.     (See  Plate  7.) 

In  preparing  the  solutions  used  in  making  the  negatives  for  this  plate 
a  fixed  amount  of  the  mother-solution  of  cobalt  chloride  in  methyl  alco- 
hol was  run  into  a  measuring-flask,  then  the  required  amount  of  water 
added,  and  finally  the  flask  was  filled  up  to  the  mark  with  pure  methyl 
alcohol.  The  concentration  of  the  colored  salt,  therefore,  remained  con- 
stant throughout.  Fourteen  solutions  were  made  up,  the  spectra  of  eight 
of  which  were  photographed  on  one  film  and  the  remaining  six  on  the 
other.  Therefore,  the  line  of  separation  of  the  two  films  falls  between  the 
eighth  and  ninth  strips,  counting  from  the  numbered  scale. 

The  concentration  of  cobalt  chloride  throughout  was  0.088.  The 
percentages  by  volume  of  water  added  were  0,  0.5,  0.75,  1,  1.25,  1.5,  2, 
3,  3.5,  4,  5,  6,  8,  and  10,  respectively.  The  strip  corresponding  to  the 
solution  in  pure  methyl  alcohol  is  adjacent  to  the  numbered  scale;  the 
strip  corresponding  to  the  solution  containing  10  per  cent  of  water  was 
at  the  top  of  the  plate.  The  depth  of  cell  was  2  cm.,  and  the  exposures 
to  the  Nernst  lamp  and  spark  were  IJ  and  3  minutes,  respectively;  the 
slit  was  0.01  cm.  wide. 

Two  regions  of  absorption  may  be  seen,  one  in  the  extreme  ultra- 
violet and  one  in  the  green.  The  two  absorption  bands  in  the  ultra-violet 
at  X  3100  and  X  3600,  already  described  for  Plate  4,  were  too  faint  to  be 
recorded  with  certainty,  and  the  merest  trace  of  the  absorption  in  the  red 
is  seen  in  the  negative  strip  nearest  the  numbered  scale.  The  band  in  the 
extreme  ultra-violet  narrows  regularly  with  addition  of  water,  the  limits 
of  transmission  for  the  solution  in  pure  alcohol  and  for  the  one  containing 
10  per  cent  of  water  being,  respectively,  X  2650  and  X  2480. 
^  The  green  band  narrows  markedly  with  increase  of  water.  Its  center 
also  shifts  slightly  towards  the  blue,  being  at  X  5250  for  the  solution  in 
pure  methyl  alcohol,  and  at  X  5210  for  the  solution  containing  the  largest 
percentage  of  water. 

The  marked  difference  between  the  ultra-violet  spectrum  shown  by 
this  plate  and  that  shown  by  Plate  23  in  "Hydrates  in  Aqueous  Solution" 
has  already  been  mentioned.      It  is  evident  from  the  negatives  that  the 


20  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

solutions  exert  considerable  absorption  in  the  ultra-violet  region,  as  the 
spectrum  is  faint  compared  with  the  comparison  spectrum.  If  this  absorp- 
tion increases  with  rise  of  temperature,  as  is  very  probable,  the  difference 
noted  might  well  be  produced  by  a  difference  in  temperature  of  only  a 
very  few  degrees. 

Cobalt  Chloride  in  Ethyl  Alcohol  with  Water.    (See  Plate  8.) 

The  concentration  of  cobalt  chloride  throughout  was  0.088.  The  per- 
centages of  water,  beginning  with  the  solution  used  in  making  the  strip 
nearest  the  comparison  spectrum,  were  0,  1,  2,  3,  4,  5,  5.5,  6,  6.5,  7,  7.5, 
8,  9,  10,  11,  and  12.  The  duration  of  the  exposures  to  the  Nernst  lamp  and 
the  spark  was,  respectively,  IJ  ai^d  3  minutes;  the  sht  was  adjusted  to  a 
width  of  0.01  cm.    The  common  depth  of  absorbing  layer  was  2.0  cm. 

The  four  solutions  containing  the  least  amounts  of  water  show  the 
bands  at  X  3100  and  X  3600  to  be  very  intense,  making  the  absorption  for 
the  solution  in  pure  ethyl  alcohol  complete  from  X  3800  to  the  end  of 
the  ultra-violet.  The  bands,  however,  disappear  rapidly  on  addition  of 
water,  so  that  in  the  strip  corresponding  to  6  per  cent  of  water  they  can 
hardly  be  noticed.  This  strip  transmits  light  as  far  out  as  X  2600,  while 
on  the  strip  made  with  the  solution  containing  12  per  cent  of  water  trans- 
mission extends  to  X  2475. 

The  band  in  the  green  behaves  very  much  the  same  as  it  does  in  methyl 
alcohol,  being,  however,  somewhat  fainter  in  ethyl  than  in  methyl  alcohol. 
Its  middle  for  the  solution  containing  12  per  cent  of  water  is  at  X  5200. 
For  the  solution  in  pure  ethyl  alcohol  it  is  not  possible  to  determine  its 
middle,  as  it  unites  with  the  strong  absorption  band  in  the  orange  and  red, 
this  solution  showing  complete  absorption  from  about  X  4930  to  X  7200. 

The  absorption  band  in  the  red  narrows  very  rapidly  with  addition  of 
water.  Its  limits  for  the  solution  containing  3  per  cent  of  water  are  X  5750 
and  X  7000.  In  the  strips  which  correspond  to  the  solutions  containing 
from  5  to  7.5  per  cent  of  water,  the  band  breaks  up  into  a  rather  compli- 
cated spectrum.  Absorption  bands  may  be  noticed  having  centers  at 
X  5910,  X  6060,  X  6240,  X  6400,  and  X  6700.  Of  these,  the  last  is  the  strong- 
est and  widest.  The  one  at  X  6400  is  very  faint,  the  one  at  X  5910  a  little 
more  intense;  while  those  at  X  6060  and  X  6240  are  fairly  intense,  compar- 
ing favorably  with  the  bands  in  the  same  region  seen  on  Plate  4. 

Referring  to  the  description  of  the  negatives  from  which  Plate  5  was 
made,  it  will  be  recalled  that,  although  the  absorption  in  the  red  showed 
signs  of  breaking  up  into  finer  bands,  these  did  not  appear  very  distinctly. 
Indeed,  the  only  ones  that  could  be  made  out  were  those  at  X  5910  and 
X  6060.  The  water  added  to  solutions  of  cobalt  chloride  in  ethyl  alcohol 
must  hence  play  an  important  part  in  the  developments  of  these  bands, 
and  it  is  barely  possible  that  the  faint  development  of  the  bands  noted 
in  Plate  5  may  have  been  due  to  slight  traces  of  water  which  it  is  impos- 
sible to  remove  from  the  ethyl  alcohol,  or  which  might  have  found  its  way 
into  the  alcohol  in  the  process  of  pouring  the  solution  into  the  cell  and  expos- 
ing this  in  the  spectrograph.  It  is  probable  that  the  activity  of  water 
in  producing  changes  in  the  absorption  spectrum  depends  not  upon  the 


SALTS    OF    COBALT.  21 

percentage  relation  of  water  to  alcohol,  but  of  water  to  amount  of  salt  in 
a  given  volume  of  the  solution.  This  was  found  to  be  so  for  solutions  of 
neodymium  salts  in  mixed  solvents,  which  will  be  discussed  in  the  chap- 
ter dealing  with  the  rare  earths.  In  the  event  of  this  rule  also  applying 
to  cobalt  salts,  we  may  say  that  since  5  per  cent  of  water  produces  a  fair 
development  of  the  bands  in  a  solution  of  concentration  0.088,  it  would 
require  only  about  1  per  cent  of  water  to  do  so  for  the  most  dilute  solu- 
tion used  in  making  the  negative  for  Plate  5,  where  the  concentration 
was  0.02.  The  bands  then  were  much  less  clearly  developed,  which  in  the 
event  of  their  being  caused  by  water  would  indicate  the  presence  of 
water  to  an  amount  of  say  0.5  per  cent,  which  is  well  within  the  range  of 
probability. 

It  is  not  at  all  improbable  that  the  absorption  bands  in  the  red  region 
of  the  spectrum,  shown  by  solutions  of  cobalt  salts  dissolved  in  ethyl  alcohol, 
could  be  used  as  a  delicate  test  for  the  presence  of  water  in  the  solvent. 

Cobalt  Chloride  in  Acetone  with  Water.    (See  Plate  9.) 

The  concentration  of  the  cobalt  salt  throughout  was  0.0108.  The 
successive  percentages  of  water  were,  beginning  with  the  solution  corre- 
sponding to  the  strip  nearest  the  numbered  scale,  0,  2,  4,  6,  8,  10,  12,  14, 
16,  18,  20,  22,  24,  26,  28,  and  30. 

The  solutions  containing  from  0  to  10  per  cent  of  water  were  deep-blue 
to  light-blue,  while  those  containing  from  16  to  30  per  cent  of  water  showed 
very  little  color  except  a  faint  suggestion  of  pink.  The  duration  of  the 
exposure  to  the  Nernst  lamp  lasted  1  minute,  that  to  the  spark  3  minutes, 
with  a  slit  width  of  0.01  cm.    The  cell  was  adjusted  to  a  depth  of  3.0  cm. 

The  intense  absorption  in  the  ultra-violet  is  due  to  the  acetone,  and 
hence  no  transmission  is  noticed  beyond  k  3300.  The  solutions  contain- 
ing the  least  amount  of  water  show  a  region  of  absorption  near  ^  3600, 
which  may  be  the  same  band  that  has  already  been  described  in  discussing 
solutions  in  the  alcohols.  The  solutions  containing  from  0  to  14  per  cent 
of  water  show  absorption  in  the  orange  and  red,  which  decreases  rapidly 
with  increase  of  the  amount  of  water. 

The  solution  in  pure  acetone  absorbs  everything  from  X  5400  to  X  7300. 
Both  edges  of  the  band  approach  each  other  rapidly  until  the  solution 
containing  10  per  cent  of  water  is  reached,  when  it  breaks  up  into  a  number 
of  narrower  bands,  their  centers  being  at  X  5910,  X  6060,  X  6240,  and  X  6700. 
The  band  at  X  6400,  in  the  ethyl  alcohol  solution  on  addition  of  water,  does 
not  appear  distinctly  enough  to  be  seen  with  certainty.  In  general,  the 
system  of  bands  is  the  same  as  that  which  is  seen  in  ethyl  alcohol,  and 
is  most  likely  due  to  the  presence  of  water;  since  it  will  be  recalled,  from 
the  description  of  Plate  6,  that  the  solution  in  pure  acetone  shows  no 
system  of  bands  at  all  comparable  with  those  here  described.  The  pro- 
portion of  water  to  salt  in  the  solution,  in  order  to  show  the  bands,  is 
very  much  larger  for  solutions  in  acetone  than  for  solutions  in  ethyl  alco- 
hol, the  values  being  about  as  15  to  1. 

The  change  in  the  spectrum  produced  by  adding  more  water  than  14 
per  cent  is  too  slight  to  be  noticed. 


22  ABSORPTION   SPECTRA    OF    SOLUTIONS. 

Cobalt  Bboahde  in  Water — Beer's  Law.    (See  Plate  10.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A  were  2.14,  1.60,  1.07,  0.71,  0.49,  0.36,  and  0.27,  the  corresponding  depths 
of  cell  being  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the  concentrations  were 
0.71,  0.53,  0.35,  0.24,  0.164,  0.12,  and  0.09;  the  depths  of  cell  were  the 
same  as  in  A.  The  strip  corresponding  to  the  most  concentrated  solu- 
tion is  in  each  case  adjacent  to  the  numbered  scale. 

The  exposures  to  the  Nernst  lamp  lasted  1^  minutes,  while  those  to 
the  spark  lasted  3  minutes,  the  slit  having  a  width  of  0.01  cm. 

The  most  concentrated  solution  had  a  deep  reddish-brown  color.  With 
dilution  the  color  gradually  changed  to  the  usual  pink.  No  exposures 
were  made  to  the  red  end  of  the  spectrum,  since  even  the  most  concen- 
trated solution  in  layers  of  2  cm.  or  more  failed  to  show  any  absorption 
in  this  region. 

The  spectrogram  shows  two  regions  of  absorption,  one  in  the  extreme 
ultra-violet  and  the  usual  one  in  the  green. 

The  most  concentrated  solution  of  set  A  transmits  the  ultra-violet 
light  of  the  spark  as  far  as  k  2720,  while  the  most  dilute  solution  trans- 
mits as  far  as  X  2570.  The  absorption  decreases  with  dilution  much  more 
rapidly  at  first,  as  is  shown  by  the  rounded  appearance  of  the  edge.  The 
most  concentrated  solution  of  B  transmits  X  2470,  while  the  most  dilute 
one  lets  through  some  light  of  wave-length  X  2350,  being,  therefore,  almost 
perfectly  transparent  to  all  wave-lengths  emitted  by  the  spark  used.  No 
trace  of  an  absorption  band  near  X  3300,  such  as  was  observed  in  aqueous 
solutions  of  cobalt  chloride,  could  be  noticed  in  the  aqueous  solutions  of 
the  bromide.  In  fact,  the  bromide  is  much  more  generally  transparent  to 
light  of  all  wave-lengths  than  the  chloride,  and  especially  so  for  the  ultra- 
violet region  of  the  spectrum.  The  band  in  the  green  behaves  very  much 
like  that  due  to  cobalt  chloride.  It  narrows  rapidly  at  first  with  dilution, 
then  more  slowly,  and  finally  remains  of  practically  constant  width  in  the 
four  most  dilute  solutions  of  set  B.  The  middle  of  the  band  in  B  is  very 
near  X  5200,  which  is  the  same  as  in  the  case  of  cobalt  chloride.  The  band 
is,  however,  considerably  narrower  in  the  bromide  than  in  the  chloride  solu- 
tions, due  in  part  to  the  slightly  smaller  concentration;  but  as  this  is  hardly 
sufficient  to  account  for  the  whole  effect  it  indicates  an  intrinsic  difiference 
in  the  behavior  of  the  two  salts. 

Cobalt  Bromide  in  Water — ^Molecules  Constant.    (See  Plate  11.) 

No  data  giving  the  dissociation  of  cobalt  bromide  were  at  hand,  and, 
accordingly,  since  it  is  a  general  rule  that  the  chlorides  and  bromides  do 
not  differ  greatly  in  this  respect,  the  dissociation  of  the  bromide  was 
assumed  to  be  the  same  as  for  cobalt  chloride.  Now  it  is  possible  that  the 
actual  value  of  the  dissociation  for  any  given  concentration  of  the  bro- 
mide might  differ  somewhat  from  the  corresponding  value  for  the  chloride, 
and  still  the  rate  of  change  of  dissociation  with  concentration  might  be 
sensibly  the  same  for  the  two  salts.  And,  evidently,  in  the  calculation  of 
the  series  of  concentrations  required  in  order  to  make  the  number  of  mole- 
cules in  the  path  of  the  beam  of  light  constant,  for  an  arbitrary  set  of 


SALTS    OF    COBALT.  23 

cell-depths,  we  are  concerned  only  with  the  rate  of  change  of  dissociation 
with  concentration,  and  not  with  the  actual  value  of  the  dissociation  for 
any  given  concentration.  A  comparison  of  the  series  of  concentrations 
fulfilling  the  condition  of  "molecules  constant"  for  various  strong  elec- 
trolytes used  in  the  present  investigation  shows  that  they  differ  from  each 
other  by  amounts  that  are  small  compared  with  experimental  errors  in 
the  measurements  of  dissociation.  Hence,  the  assumption  made  in  the 
present  case  would  not  introduce  any  sensible  error. 

The  concentrations  used  in  making  the  negative  for  A  were  1.53,  1.14, 
0.885,  0.695,  and  0.570;  the  corresponding  depths  of  absorbing  layer  of 
solution  were  6,  9,  13,  18,  and  24  mm.,  respectively.  For  B  the  con- 
centrations were  0.70,  0.575,  0.434,  0.329,  0.253,  0.197,  and  0.158;  the  cor- 
responding depths  of  cell  were  3,  4,  6,  9,  13,  18,  and  24  mm.  It  will  be 
noticed  that  the  concentrations  for  B,  as  well  as  the  depths  of  cell,  are 
identical  with  those  used  in  making  the  negative  for  Plate  3,  and,  hence, 
a  comparison  of  the  two  sets  of  spectra  shows  at  a  glance  the  relative 
absorbing  power  of  cobalt  bromide  and  cobalt  chloride  solutions.  The 
latter  exerts  a  considerably  greater  absorbing  power  throughout  the 
spectrum. 

The  exposure  to  the  Nernst  filament  and  spark  for  both  A  and  B  lasted 
respectively  1^  and  3  minutes,  the  width  of  slit  being  0.01  cm.  as  usual. 
The  spectrogram  shows  that  the  absorption  in  the  extreme  ultra-violet 
remains  constant,  the  limit  of  transmission  in  A  being  at  X  2700,  in  B  at 
X  2470.  The  band  in  the  green  widens  with  dilution  throughout,  and 
quite  uniformly,  differing  in  this  respect  from  the  chloride,  where  it  will 
be  recalled  that  the  band  at  first  remained  of  constant  width,  and  then 
began  to  widen  with  dilution.  No  exposure  to  the  red  end  of  the  spectrum 
was  made  at  all;  the  solutions  transmitting  freely  light  of  all  wave-lengths 
between  X  5700  and  X  7600. 

Cobalt  Bromide  with  Calcium  Bromide.     (See  Plate  12.) 

The  concentration  of  cobalt  bromide  throughout  was  constant  and 
equal  to  0.260.  The  concentrations  of  calcium  bromide,  beginning  with 
the  solution  corresponding  to  the  strip  adjacent  to  the  numbered  scale, 
were  3.73,  3.56,  3.39,  3.22,  3.05,  2.88,  2.72,  2.54,  2.37,  2.20,  1.87,  1.53, 
1.19,  0.85,  0.51,  0.00.  The  differences  in  concentration  were,  accord- 
ingly, 0.17,  0.17,  0.17,  0.17,  0.17,  0.17,  0.17,  0.17,  0.17,  0.34,  0.34,  0.34, 
0.34,  0.34,  0.51. 

The  solution  containing  the  greatest  amount  of  calcium  bromide  was 
purplish-blue  in  color,  from  which  the  color  changed  to  the  reddish-pink 
of  cobalt  bromide  in  the  solution  containing  no  calcium  salt. 

The  cell  was  adjusted  to  a  depth  of  1.1  cm.  for  each  of  the  solutions; 
the  exposures  to  the  Nernst  lamp  and  spark  being,  respectively,  1^  and 
3  minutes.  The  solution  containing  no  calcium  bromide  transmits  all 
wave-lengths  from  X  2400  to  X  7600,  with  the  exception  of  the  region 
X  5000  to  X  5300  which  is  absorbed.  With  addition  of  calcium  bromide  the 
band  in  the  extreme  ultra-violet  widens  rapidly,  so  that  in  the  solution 
pertaining  to  the  tenth  strip,  counting  from  the  scale,  all  radiations  of 


24  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

shorter  wave-length  than  X  2800  are  absorbed.  In  addition  an  absorption 
band  having  its  center  at  X  3010  makes  its  appearance,  and  increases  rap- 
idly in  intensity  and  width  with  the  increase  in  the  amount  of  calcium 
bromide.  In  the  solution  corresponding  to  the  eleventh  strip,  this  band 
and  the  band  in  the  extreme  ultra-violet  have  run  together,  causing  all 
light  of  wave-length  shorter  than  X  3140  to  be  absorbed. 

The  question  whether  all  the  absorption  in  the  ultra-violet  is  due  to 
the  presence  of  calcium  bromide  as  such,  or  whether  it  is  due  to  some 
action  of  this  salt  on  the  cobalt  bromide  in  the  solution  may  be  answered 
by  referring  to  Plate  11  A,  in  "Hydrates  in  Aqueous  Solution,"  and  to 
the  description  of  it  given  on  page  208,  as  well  as  to  Plate  10,  which  shows 
the  effect  on  the  absorption  of  cobalt  chloride  produced  by  adding  cal- 
cium bromide  of  various  concentrations.  A  concentrated  solution  of 
calcium  bromide  4.236  normal,  in  a  depth  of  layer  of  1.41  cm.,  still  trans- 
mits some  light  of  wave-length  shorter  than  X  2800,  although  it  exerts 
some  absorption  as  far  as  X  3100.  Plate  10,  "Hydrates  in  Aqueous  Solu- 
tion," shows  that  a  solution  of  cobalt  chloride  containing  calcium  bromide 
in  such  concentrations  as  0.7  normal,  still  transmits  light  of  a  wave-length 
as  short  as  X  2500  when  the  layer  is  1.41  cm.  deep,  without  any  sign  of 
absorption  in  the  region  X  3010.  We  may  accordingly  conclude  that  the 
widening  of  the  extreme  ultra-violet  band  in  Plate  12  is  due  to  the  absorb- 
ing action  of  calcium  bromide,  but  that  the  band  at  X  3010  is  to  be  ascribed 
to  the  joint  action  of  this  salt  and  cobalt  bromide. 

The  band  in  the  green  widens  regularly  and  symmetrically  with  the 
addition  of  calcium  bromide.  Measurements  on  the  negatives  give  the 
following  results:  For  the  strip  pertaining  to  the  solution  containing  no 
dehydrating  agent,  the  limits  of  transmission  are  X  5020  and  X  5300,  center 
at  X  5160.  For  the  strip  corresponding  to  the  solution  containing  the 
greatest  amount  of  calcium  bromide  the  limits  were  X  4770  and  X  5560, 
center  at  X  5165. 

In  the  red  the  solutions  containing  the  greatest  amount  of  the  dehy- 
drating agent  show  a  set  of  four  absorption  bands  having  their  centers 
at  X  6400,  X  6650,  X  6950;  the  center  of  the  fourth  band  lies  in  the  extreme 
red,  beyond  the  region  of  sensibility  of  the  photographic  plate  used.  Its 
presence  is  shown  by  the  cutting  off  of  the  extreme  red  end  of  the  strips 
nearest  the  numbered  scale.  The  band  at  X  6400  is  rather  narrow,  the 
others  moderately  wide,  the  one  at  X  6650  being  the  most  intense  and  also 
persisting  the  longest  with  decrease  in  the  amount  of  the  calcium  salt. 
A  comparison  of  this  spectrum  with  the  diagram  shown  on  page  197  of 
"Hydrates  in  Aqueous  Solution,"  which  shows  the  position  and  relative 
intensity  of  the  red  bands  of  cobalt  chloride  brought  out  by  adding  large 
quantities  of  calcium  chloride,  reveals  some  notable  differences.  The  posi- 
tions of  the  bands  are  sensibly  the  same,  but  in  calcium  chloride  the  band 
at  X  6950  is  stronger  than  the  one  at  X  6650,  whereas  the  reverse  is  true 
for  the  bromide.  Also,  the  chloride  shows  a  weak  band  at  X  6400  with 
stronger  ones  at  X  6240,  and  X  6095,  while  the  bromide  has  an  intense  band 
at  X  6400,  and  none  at  all  in  the  other  positions.  The  bands  with  the 
chloride  were  the  same  whether  calcium  chloride,  calcium  bromide,  or 


SALTS    OF    COBALT.  26 

aluminium  chloride  was  used  as  a  dehydrating  agent,  thus  indicating 
that  the  difference  in  the  spectra  which  we  have  just  considered  is  to 
be  referred  to  the  colored  salt  and  not  to  the  dehydrating  agent. 

Cobalt  Bkomide  in  Methyl  Alcohol — Beer's  Law.    (See  Plate  13.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
B  were  0.228,  0.191,  0.158,  0.130,  0.108,  0.091,  and  0.076;  the  corre- 
sponding depths  of  absorbing  layer  were  8,  9.5,  11.5,  14,  17,  20,  and  24 
mm.  For  set  A  the  concentrations  were  0.114,  0.096,  0.079,  0.065,  0.054, 
0.045,  and  0.038.  The  depths  of  absorbing  layer  were  the  same  as  for  set 
B.  The  concentrations  were  accordingly  about  9  per  cent  smaller  for  B 
than  was  the  case  in  the  similar  set  for  cobalt  chloride,  and  about  18  per 
cent  smaller  in  case  of  A. 

The  most  concentrated  solution  was  slightly  purplish  in  color,  from 
which  the  color  changed  to  the  usual  pink  of  cobalt  solutions.  The  con- 
centrated solutions  had  very  httle  general  absorption,  differing  in  this 
respect  markedly  from  those  of  cobalt  chloride  in  the  same  solvent.  The 
exposures  to  the  Nernst  lamp  and  spark  were,  respectively,  1^  and  3 
minutes,  the  slit  being,  as  usual,  0.01  cm.  in  width. 

The  edge  of  the  ultra-violet  band  is  perfectly  straight  in  both  sets, 
showing  that  Beer's  law  holds  rigidly.  The  limit  of  transmission  in  B 
is  X  2850,  while  for  A  it  is  >i  2760,  thus  indicating  much  stronger  absorp- 
tion in  this  region  for  the  bromide  than  for  the  chloride.  The  chloride, 
although  transmitting  some  light  of  shorter  wave-length,  does,  however, 
exert  a  greater  amount  of  absorbing  action  in  the  region  below  X  3700, 
since  it  has  two  bands,  one  at  X  3100  and  one  at  X  3600,  both  of  which  are 
entirely  absent  in  the  bromide  solution.  The  band  in  the  green  is  not  as 
intense  as  in  the  chloride  solution.  Its  center  is  at  X  5250,  and  it  narrows 
slightly  with  dilution  in  both  B  and  A. 

The  slight  absorption  in  the  red,  which  was  undoubtedly  present  in 
the  most  concentrated  solution,  was  not  sufficient  to  be  registered  on  the 
photographic  plate.  This,  accordingly,  indicates  complete  transparency 
from  the  green  band  to  beyond  X  7400. 

Cobalt  Bromide  in  Ethyl  Alcohol — Beer's  Law.     (See  Plate  14.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A 
were  0.146,  0.122,  0.102,  0.084,  0.069,  0.058,  and  0.044,  the  depths  of  cell 
being  8,  9.5,  11.5,  14,  17,  20,  and  24  mm.  The  concentrations  for  B  were 
0.058,  0.049,  0.040,  0.033,  0.026,  0.023,  and  0.019;  the  depths  of  cell  were 
the  same  as  for  A.  The  concentrations  were  accordingly  almost  the  same 
throughout  as  those  used  in  making  the  negative  for  Plate  5,  so  that  the 
two  are  directly  comparable. 

All  the  solutions  were  blue,  the  color  becoming  very  faint,  however, 
in  the  most  dilute  solutions  of  set  B.  Exposures  and  slit-width  were  the 
same  as  for  Plate  13. 

There  is  strong  absorption  in  the  extreme  ultra-violet  which,  how- 
ever, narrows  slightly  with  dilution;  the  limits  of  transmission  being 
X  3060  and  X  3000  for  the  most  concentrated  and  most  dilute  solutions  of 


26  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

A,  respectively,  and  X  2920  and  X  2820  for  those  in  B.  No  trace  of  any 
bands  at  X  3100  or  X  3600,  as  found  in  the  corresponding  solutions  of  cobalt 
chloride,  can  be  seen.  The  green  band  may  be  seen  in  A,  but  unlike  the 
chloride  solution  there  is  a  region  of  complete  transparency  on  the  red 
side  of  it.  In  B  the  green  band  is  so  extremely  weak  that  the  slight  shad- 
ing noticeable  near  X  5250  may  be  due  to  the  lack  of  sensibility  of  the 
Seed  film  in  this  region. 

In  A  there  is  a  wide  region  of  complete  absorption,  which  narrows 
rapidly  with  dilution,  breaking  up  into  two  absorption  bands  in  B.  The 
limits  of  transmission  for  the  most  concentrated  solutions  of  A  are  X  5750 
and  X  7200,  while  for  the  most  dilute  solutions  the  figures  are  A  6150  and 
X  6900.  It  appears,  therefore,  that  the  red  edge  of  the  absorption  band  is 
a  little  nearer  the  region  of  long  wave-lengths  than  is  the  case  for  the  cor- 
responding solutions  of  cobalt  chloride. 

B  shows  two  absorption  bands  having  their  centers  at  X  6160  and  X  6730, 
respectively.  Both  narrow  rapidly  with  dilution,  disappearing  practically 
in  the  most  dilute  solution.  No  trace  of  the  narrower  bands  seen  in  the 
chloride  solution  is  visible. 

Cobalt  Bromide  in  Acetone — Beer's  Law.    (See  Plate  15.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A  varied  from  0.037  to  0.012;  the  depths  of  absorbing  layer  were  8,  9.5, 
11.5,  14,  17,  20,  and  24  mm.  In  B  the  concentrations  were  varied  from 
0.012  to  0.004,  the  depths  of  cell  used  being  the  same  as  in  set  A.  The 
strip  corresponding  to  the  most  concentrated  solution  is  in  each  case 
adjacent  to  the  numbered  scale. 

The  solutions  were  all  blue,  only  the  intensity  of  the  color  varying 
with  dilution.  The  exposures  to  the  Nernst  lamp  and  spark  were,  respec- 
tively, li  and  2^  minutes.  The  absorption  in  the  ultra-violet  is  of  course 
to  be  ascribed  to  the  solvent,  and  has  exactly  the  same  Umits  as  given 
under  Plate  6. 

In  the  red  is  a  wide  absorption  band  which  in  set  A  extends  farther 
into  the  red  than  the  panchromatic  plate  is  capable  of  recording.  The 
violet  edge  of  the  band  lies  at  X  5750  and  is  fairly  sharp.  The  red  edge 
was  shown  by  visual  observations  to  lie  very  close  to  X  7600.  In  B  the 
band  has  narrowed  considerably,  as  some  light  is  transmitted  as  far  up  in 
the  red  as  X  6200,  the  red  edge  being  at  approximately  X  7050.  The  violet 
edge  of  the  band  is  very  hazy,  the  transmission  being  greatly  weakened 
as  far  towards  the  violet  as  X  5850.  The  absorption  shows  no  indication 
of  breaking  up  into  smaller  bands  as  did  that  of  cobalt  chloride  in  acetone. 
Beer's  law  appears  to  hold  very  accurately,  the  absorption  shown  by  all 
the  strips  of  one  set  being  the  same. 

In  comparing  this  plate  with  Plate  6,  it  should  be  borne  in  mind  that 
the  solutions  used  in  making  this  plate  had  almost  double  the  concen- 
tration used  in  the  other  case.  This  accounts  for  the  somewhat  stronger 
absorption  shown  by  the  bromide.  The  indications  are,  however,  that  the 
absorbing  power  of  the  bromide  in  acetone  is  somewhat  smaller  than  that  of 
the  chloride,  which  agrees  with  what  has  been  noticed  in  the  other  solvents. 


SALTS    OF    COBALT.  27 

Cobalt  Bromide  in  Methyl  Alcohol  with  Water.    (See  Plate  16.) 

The  concentration  of  cobalt  bromide  throughout  was  constant  and 
equal  to  0.088.  The  percentages  of  water,  beginning  with  the  solution 
whose  spectrum  is  adjacent  to  the  numbered  scale,  were  0,  0.5,  0.75,  1, 
1.25,  1.5,  2,  2.5,  3,  3.5,  4,  5,  6,  8,  10,  12. 

The  solution  containing  no  water  was  slightly  purplish  in  color,  from 
which  the  color  changed  rapidly  with  addition  of  water  to  a  pink.    After 

2  per  cent  of  water  had  been  added  no  appreciable  change  in  color  could 
be  noticed  by  the  unaided  eye.  The  depth  of  absorbing  layer  used  through- 
out was  2.0  cm.,  the  exposures  to  the  Nernst  lamp  and  spark  being  1  and 

3  minutes,  respectively. 

The  solution  containing  no  water,  and  the  one  containing  0.5  per  cent 
of  water,  show  considerable  absorption  from  X  3900  towards  the  ultra- 
violet, which  at  first  sight  does  not  agree  very  well  with  what  is  shown 
by  B,  Plate  13.  The  discrepancy,  however,  disappears  when  it  is  noted 
that  when  the  series  of  exposures  for  the  spectrogram  now  under  discus- 
sion was  made,  the  workroom  was  at  a  temperature  of  about  30°  C;  whereas 
when  the  negatives  for  Plate  13  were  made  the  temperature  was  about 
20°  C.  Cobalt  bromide  dissolved  in  methyl  or  ethyl  alcohol  is  very  sensitive 
to  temperature  change,  a  pink  solution  of  proper  concentration  turning  pur- 
ple with  a  comparatively  slight  change  of  temperature.  From  the  solution 
containing  0.75  per  cent  of  water  to  the  one  containing  the  largest  amount 
of  water,  the  ultra-violet  absorption  recedes  gradually  from  X  2850  to  X  2580. 

The  absorption  band  in  the  green  is  somewhat  narrower  than  in  the 
solution  of  the  chloride,  but  like  that  it  narrows  regularly  with  addition 
of  water.  Measurements  on  the  edges  of  the  band  in  the  strip  pertaining 
to  the  solution  containing  no  water,  give  the  wave-lengths  X  5050  and  X  5400, 
locating  the  center  of  the  band  at  X  5225.  For  the  strip  pertaining  to  the 
solution  containing  12  per  cent  of  water  the  figures  are  ^5100  and  X  5300, 
locating  the  center  at  X  5200.  The  center,  therefore,  shifts  slightly  towards 
the  shorter  wave-lengths  with  addition  of  water. 

The  absorption  in  the  red  was  not  of  sufficient  intensity  to  be  regis- 
tered on  the  photographic  plates,  which,  accordingly,  show  uniform  and 
complete  transmission  from  X  5500  to  X  7400. 

Cobalt  Bromide  in  Ethtl  Alcohol  with  Water.    (See  Plate  17.) 

The  concentration  of  cobalt  bromide  was  constant  throughout  and 
equal  to  0.088.  The  percentages  of  water,  beginning  with  the  solution 
whose  spectrum  is  adjacent  to  the  numbered  scale,  were  0,  1,  1.5,  2,  2.5, 
3,  3.5,  4,  5,  6,  7,  7.5,  8,  9,  10,  and  11. 

The  solution  containing  no  water  was  blue  or  purplish-blue;  the  one 
containing  1  per  cent  of  water  was  slightly  purplish,  from  which  the  color 
changed  very  rapidly  to  the  regular  pink  of  cobalt  solutions.  No  color 
change  appreciable  to  the  eye  was  noticed  after  the  solution  containing 
3  per  cent  of  water  was  reached. 

The  cell  was  adjusted  to  a  depth  of  2.0  cm.  and  the  exposures  to  the 
Nernst  lamp  and  spark  were,  respectively,  of  1  and  3  minutes  duration; 
the  slit  was,  as  usual,  0.01  cm.  wide.    The  solution  containing  no  water 


28  ABSORPTION    SPECTRA    OF   SOLUTIONS. 

absorbs  all  light  of  shorter  wave-length  than  X  3000.  The  absorption 
recedes  gradually  with  addition  of  water,  and  a  Httle  more  rapidly  at  first, 
as  is  apparent  from  the  curvature  of  the  edge  of  the  band  nearest  the  scale. 
The  solution  containing  11  per  cent  of  water  transmits  light  of  a  wave- 
length as  short  as  X  2560.  The  green  band  has  very  nearly  the  same  inten- 
sity as  it  had  in  the  methyl  alcohol  solutions  just  described,  and  narrows 
with  addition  of  water  to  about  the  same  extent.  Measurements  give  its 
center  for  the  solution  containing  no  water  at  X  5220,  and  for  the  solution 
containing  11  per  cent  of  water  at  X  5200. 

The  absorption  in  the  red  is  of  a  general  character,  showing  only  faint 
indications  of  bands.  The  whole  spectrum  of  the  solution  containing  no 
water  is  weak  throughout  the  entire  red  region,  and  shows  weak  bands 
superposed  upon  the  general  absorption,  having  their  centers  at  about 
^6150  and  X  6730.  These  regions  correspond  roughly  to  the  minima  of 
sensibility  of  the  panchromatic  plates ;  therefore,  due  allowance  must 
be  made  for  this  in  studying  the  spectrogram.  The  absorption  decreases 
rapidly  with  increase  in  the  amount  of  water,  being  practically  absent  in 
the  solution  containing  2  per  cent.  All  solutions  containing  a  greater 
percentage  of  water  are  completely  transparent  to  the  red  as  far  as  the 
limit  of  sensibility  of  the  plates  used. 

Cobalt  Bromide  in  Acetone  with  Water.    (See  Plate  18.) 

The  concentration  of  the  colored  salt  throughout  was  constant  and 
equal  to  0.007  normal.  The  successive  percentages  of  water,  beginning 
with  the  solution  whose  spectrum  is  nearest  the  numbered  scale,  were 
0,  1,  2,  3,  4,  5,  6,  7,  8,  9,  10,  11,  12,  13,  14,  and  15. 

The  first  seven  solutions  were  of  various  shades  of  blue,  from  dark  to 
light-blue.  The  eighth,  ninth,  and  tenth  were  light-bluish  to  bluish-green, 
after  which  the  solutions  were  almost  colorless,  a  very  faint  pinkish  tinge 
only  being  noticed. 

The  depth  of  absorbing  layer  used  was  2.0  cm.,  and  the  exposures  to 
the  Nernst  lamp  and  spark  lasted  for  1  and  3  minutes,  respectively,  the 
slit  being,  as  usual,  0.01  cm.  wide. 

As  has  been  repeatedly  observed,  acetone  absorbs  all  wave-lengths 
shorter  than  about  X  3300  for  the  thickness  of  layer  here  used.  The  edge 
of  the  band  is  unusually  sharp,  which  is,  however,  not  the  case  for  the 
first  eight  strips  of  Plate  18.  In  fact  the  strip  nearest  the  scale  just  barely 
records  the  line  at  X  3320,  the  shading  from  this  point  towards  X  3800  being 
considerable.  This  shading  is  undoubtedly  due  to  the  effect  of  the  colored 
salt.  It  decreases  with  addition  of  water,  but  is  still  easily  noticeable jn 
the  solution  containing  15  per  cent  of  water. 

Measurements  on  the  edges  of  the  band  in  the  red  settings  being  made 
on  the  limits  of  transmission,  give  the  following: 

Solution  which  contains  no  water,  A  6600  to  beyond  ^  7400 
Solution  containing  1  p.  ct.  of  water,  A  5680  to  beyond  A  7400 
Solution  containing  2  p.  ct.  of  water,  A  5750  to  beyond  A  7400 
Solution  containing  3  p.  ct.  of  water,  A  5850  to  A  7400 
Solution  containing  4  p.  ct.  of  water,  A  5970  to  A  7350 
Solution  containing  5  p.  ct.  of  water,  A  6070  to  A  7250 
Solution  containing  6  p.  ct.  of  water,  absorption  band  broken  up. 


f 


I 


SALTS    OF   COBALT.  29 

Bands  are  seen  having  their  centers  at  X  6160  and  X  6350,  with  a  band 
extending  from  X  6650  to  X  7150.  In  the  solution  containing  7  per  cent 
of  water  the  latter  band  is  broken  up  into  two,  with  centers  at  X  6700  and 
X  7000.  The  indications  are  that  the  band  having  its  center  at  X  6350  is 
really  made  up  of  two  bands,  the  centers  being  at  X  6300  and  X  6430,  respec- 
tively. They  are,  however,  very  faint  and  the  slightly  greater  transpar- 
ency near  X  6360  not  very  well  marked.  The  three  solutions  containing 
the  least  amount  of  water  were  examined  with  a  small,  direct-vision, 
prism  spectroscope,  having  a  scale  attached  so  that  wave-lengths  could 
be  read  ofif  directly.  These  solutions  were  all  transparent  in  the  extreme 
red,  the  edge  of  the  absorption  band  for  the  solution  containing  no  water 
being  between  X  7500  and  X  7600.  It  seemed  extremely  sharp,  but  this 
was  undoubtedly  due  to  the  small  dispersion  of  a  prism  spectroscope  in 
this  region  of  the  spectrum. 

Here  again,  as  in  the  case  of  cobalt  chloride  in  ethyl  alcohol  and  water, 
we  must  assume  that  the  system  of  narrow  absorption  bands  is  due  in 
some  way  to  the  presence  of  the  water,  for  the  absorption  of  cobalt  bro- 
mide in  acetone  showed  no  sign  of  breaking  up  into  finer  bands  with  dilu- 
tion. The  group  of  bands  brought  out  in  the  bromide  solution  is  quite 
different  from  that  shown  by  the  corresponding  solution  of  the  chloride, 
a  fact  which  must  be  accounted  for  by  any  theory  of  the  cobalt  solutions. 
It  will  be  recalled  also  that  water  added  to  solutions  of  cobalt  bromide  in 
ethyl  alcohol  did  not  break  up  the  absorption  into  finer  bands,  as  was  the 
case  with  the  chloride  solution  under  similar  circumstances. 

Cobalt  Nitrate  in  Water — Beer's  Law.    (See  Plate  19.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.05,  1.53,  1.02,  0.683,  0.473,  0.342,  and  0.256;  the  correspond- 
ing depths  of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.  For  set 
B  the  concentrations  were  0.683,  0.513,  0.342,  0.227,  0.158,  0.114,  and 
0.085;  the  depths  of  absorbing  layer  were  the  same  as  for  A. 

The  most  concentrated  solutions  were  purple  in  color,  from  which  the 
color  changed  to  the  usual  pink  with  dilution. 

The  exposures  to  the  Nernst  lamp  and  spark  lasted  1  and  3  minutes, 
respectively,  the  width  of  the  slit  used  being,  as  usual,  0.01  cm. 

The  spectrogram  shows  two  regions  of  absorption,  one  in  the  ultra- 
violet and  one  in  the  green.  In  A  the  edge  of  the  ultra-violet  band  is  per- 
fectly straight  and  sharp,  reminding  one  of  the  absorption  due  to  acetone; 
the  limit  of  transmission  being  at  X  3280,  which  is  also  the  same  as  for  ace- 
tone. In  B  the  corresponding  edge  falls  at  X  3200,  but  here  we  also  find 
a  region  of  transmission  located  at  about  X  2650,  increasing  in  width  with 
dilution.  A  band  of  absorption  is  thus  outlined,  the  limits  of  which  are 
X  2650  and  X  3200.  A  number  of  spectrograms  of  nitrates  shows  that  this 
band  is  always  present.  It  is  hence  connected  in  some  way  with  the  radi- 
cal NO3,  but  B  makes  it  seem  improbable  that  it  is  due  to  the  free  NO3  ion, 
since  the  number  of  these  in  the  path  of  the  beam  of  light  was  increasing 
in  the  direction  away  from  the  numbered  scale,  while  the  absorption  band 


30  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

narrows  somewhat  from  strip  to  strip  in  this  direction.  The  region  of 
transparency  beyond  this  NO3  band,  as  we  shall  designate  it,  shows  that 
the  ultra-violet  absorption  of  cobalt  nitrate  decreases  with  increasing 
dilution,  when  the  conditions  for  Beer's  law  obtain.  If  it  were  not  for 
the  NO3  band  we  should  expect  the  ultra-violet  end  of  transmission  to 
look  very  much  as  it  did  in  the  case  of  cobalt  bromide,  except  that  in  the 
present  case  it  would  be  shifted  somewhat  towards  the  longer  wave-lengths. 

The  absorption  band  in  the  green  presents  a  different  appearance  from 
what  it  has  done  in  the  solutions  already  studied.  In  both  the  chloride 
and  bromide  solutions  the  band  narrowed  rapidly  at  first,  and  almost 
symmetrically.  Here  it  narrows  uniformly  from  strip  to  strip  in  A,  and 
much  more  from  the  violet  than  from  the  red  side.  The  result  is  that  if 
measurements  are  made  on  the  edges  and  calculations  made  for  the  posi- 
tion of  the  center  of  the  band,  this  shifts  continually  towards  the  red  with 
increasing  dilution.  The  extreme  limits  of  transmission  in  the  most  con- 
centrated solution,  as  shown  by  the  strip  nearest  the  scale,  are  X  4450  and 
X  5500,  locating  the  center  at  X  4975.  For  the  most  dilute  solution  of  set  A 
the  corresponding  numbers  are  X  4700  and  X  5450,  center  at  X  5075.  In  set 
B  the  band  remains  of  sensibly  constant  width,  its  center  falling  at  X  5150. 

The  concentrations  used  in  making  this  spectrogram  were  almost  exactly 
the  same  as  those  employed  in  making  the  negatives  for  Plate  10,  hence 
the  two  spectrograms  are  directly  comparable.  The  comparison  shows 
that  for  solutions  of  1.5  to  2.0  normal  concentration,  the  width  of  the  band 
is  approximately  the  same  for  the  two  salts;  the  band  in  the  nitrate  solu- 
tion was,  however,  located  nearer  the  region  of  short  wave-lengths.  As 
the  concentration  decreases  the  absorption  of  the  bromide  solution  becomes 
much  less  than  that  of  the  nitrate  solution,  the  band  in  the  latter  being 
still  slightly  more  refrangible. 

The  solutions  transmitted  the  red  as  far  as  X  7400  without  sensible 
absorption.  Since  the  most  concentrated  solutions  looked  purple  in  the 
bottles,  it  is  probable  that  they  exerted  some  general  absorption  in  the 
orange  and  red,  but  examination  of  the  light  transmitted  through  the 
bottles,  where  the  layer  was  5  cm.  in  depth,  with  a  direct-vision  spectro- 
scope failed  to  reveal  any  definite  bands  in  this  region. 

Cobalt  Nitrate  in  Water — Molecules  Constant.    (See  Plate  20.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.05,  1.67,  1.25,  0.94,  0.71,  0.55,  and  0.44,  the  corresponding 
depths  of  absorbing  layer  being  3,  4,  6,  9,  13,  18,  and  24  mm.  The  con- 
centrations for  set  B  were  0.55,  0.44,  0.32,  0.23,  0.17,  0.13,  0.105;  the 
depths  of  cell  were  the  same  as  for  A.  The  width  of  slit  and  exposures 
were  the  same  as  in  the  case  of  Plate  19. 

In  A  the  ultra-violet  transmission  is  limited  by  the  red  edge  of  the 
NO3  band.  In  the  present  case  this  band  widens  somewhat  with  decreas- 
ing concentration,  the  limits  of  transmission  being  X  3280  and  X  3330  for 
the  most  concentrated  and  most  dilute  solutions,  respectively.  In  B  there 
is  some  transmission  beyond  the  NO3  band,  the  violet  limit  of  the  trans- 


SALTS    OF    COBALT.  31 

mitted  region  remaining  unchanged  with  dilution.  This  is  exactly  the 
same  as  was  found  with  cobalt  bromide  when  molecules  were  kept  con- 
stant. This  ultra-violet  absorption  does  not  vary  with  dilution,  indicat- 
ing that  it  might  be  a  property  of  the  molecules  of  the  dissolved  salt.  The 
NO3  band,  however,  widens  quite  perceptibly  with  dilution,  its  limits 
(transmission)  in  the  most  concentrated  solution  of  B  being  X  2800  and 
A  3150,  while  the  corresponding  figures  for  the  most  dilute  solution  are 
X  2700  and  X  3230. 

The  green  band  in  A  widens,  though  not  uniformly,  with  decrease  in 
concentration.  The  edges  for  the  most  concentrated  solution  are  at  X  4550 
and  X  5500,  and  for  the  most  dilute  solution  X  4450  and  X  5600,  respectively. 
It  appears,  therefore,  that  for  this  range  of  concentrations  the  widening 
is  symmetrical,  the  center  of  the  band  remaining  approximately  at  X  5000. 
In  B  the  band  also  widens,  but  somewhat  unsymmetrically,  the  violet 
edge  being  displaced  much  more  than  the  red  edge.  The  center  of  the 
band  for  the  most  dilute  solution  is  at  X  5175.  The  red  is  freely  transmitted 
to  beyond  X  7400. 

Cobalt  Sulphate  in  Water — Beeb's  Law.     (See  Plate  21.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.65,  0.52,  0.41,  0.33,  0.26,  0.20,  and  0.16;  the  corresponding 
depths  of  absorbing  layer  were  6,  7.5,  9.5,  12,  15,  19,  and  24  mm.  For 
set  B  the  concentrations  were  0.26,  0.20,  0.16,  0.13,  0.10,  0.08,  and  0.06; 
the  depths  of  cell  were  the  same  as  for  set  A.  The  slit  was  adjusted  to  a 
width  of  0.01  cm.  and  the  exposures  to  the  Nernst  lamp  and  spark  lasted 
for  1  and  2  minutes,  respectively. 

The  solutions  have  only  one  absorption  band,  namely  the  one  in  the 
green.  In  the  ultra-violet  they  are  perfectly  transparent,  the  last  lines 
in  the  comparison  spectrum  showing  with  the  same  intensity  through 
the  cell  containing  the  solution.  In  the  red  they  transmit  freely  light  of 
all  wave-lengths  as  far  up  as  X  7400. 

The  band  in  the  green  has  exactly  the  same  width  and  position  in  all 
the  strips  belonging  to  the  solutions  of  one  set;  hence.  Beer's  law  holds 
rigidly.  In  set  A  the  extreme  limits  of  transmission  are  X  4900  and  X  5400. 
The  shading  of  the  violet  edge  is  great,  extending  somewhat  below  X  4500. 
The  red  edge  is  somewhat  sharper,  although  not  as  much  so  as  would  ap- 
pear from  the  spectrogram.  The  increasing  sensibility  of  the  Seed  film 
with  wave-length  in  the  region  of  X  5400  makes  this  edge  appear  much 
sharper  than  it  really  is. 

A  comparison  of  the  seventh  strip  (counted  from  the  scale)  of  A  with 
the  seventh  strip  of  B,  Plate  3,  shows  that  for  a  concentration  of  0. 16  nor- 
mal, the  absorption  of  the  sulphate  is  slightly  greater  than  for  the  chloride; 
as  the  chloride  deviates  from  Beer's  law,  while  the  sulphate  solutions  do 
not.  We  may  say  that,  for  a  certain  range  of  concentrations  greater  than 
0.2  normal,  the  absorbing  power  of  solutions  of  the  chloride  and  sulphate 
is  the  same  in  the  green;  for  greater  concentrations  the  chloride  exerts 
stronger  absorption,  while  for  more  dilute  solutions  the  absorption  of  the 


32  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

sulphate  solutions  is  greater.  A  comparison  of  Plate  19  B  shows  that  the 
absorption  band  in  the  green  is  of  sensibly  the  same  width,  while  the  con- 
centration of  the  nitrate  solution  pertaining  to  the  seventh  strip  was  about 
40  per  cent  greater  than  that  of  the  corresponding  sulphate  solution.  As 
the  sulphate  dissociates  less  strongly  than  the  nitrate  or  chloride,  it  does  not 
seem  altogether  reasonable  to  ascribe  the  green  band  to  the  cobalt  ions. 

Since  Beer's  law  holds  so  accurately  for  the  sulphate  solutions,  it  was 
evidently  unnecessary  to  study  sets  of  solutions  of  such  concentrations 
as  to  give  a  constant  number  of  ions,  or  a  constant  number  of  molecules. 
In  the  former  case  we  should  get  a  marked  narrowing  of  the  band  and  in 
the  latter  case  a  widening. 

Cobalt  Sulphoctanate  in  Water — Beek's  Law.    (See  Plate  22.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.17,  1.62,  1.08,  0.72,  0.50,  0.36,  and  0.27,  the  corresponding 
depths  of  cell  being  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the  concentrations 
were  0.36,  0.27,  0.18,  0.12,  0.083,  0.060,  and  0.045;  the  depths  of  absorb- 
ing layer  were  the  same  as  in  set  A. 

The  color  of  solutions  of  the  sulphocyanate  is  much  deeper  than  of 
solutions  of  other  salts  of  cobalt  used  in  the  present  work.  The  most  con- 
centrated solutions  were  very  deep  purple,  from  which,  as  concentration 
decreased,  the  color  changed  to  the  usual  pink.  The  exposures  to  the 
Nernst  lamp  and  spark  were,  respectively,  1^  and  3  minutes,  the  slit  being 
adjusted  to  a  width  of  0.01  cm. 

The  spectrogram  shows  a  region  of  strong  absorption  in  the  ultra- 
violet, which  narrows  rapidly  with  dilution,  the  limit  of  transmission  for 
the  most  concentrated  solution  in  A  being  X  3630,  while  for  the  most  dilute 
solution  it  is  X  3370.  The  corresponding  wave-lengths  for  B  are  X  3240 
and  >l  3130. 

The  absorption  band  in  the  green  is  very  wide  in  A,  extending  in  the 
case  of  the  most  concentrated  solution  from  X  4300  to  X  6350,  and  in  the 
most  dilute  from  X  4350  to  X  5700.  The  violet  edge  moves  towards  the  red 
slowly  but  uniformly  with  dilution,  while  the  red  edge  moves  in  the  oppo- 
site direction  very  rapidly  from  the  first  to  the  third  solution,  then  more 
slowly.  There  is  apparently  a  band  in  the  red  not  resolved  from  the  green 
band,  which  disappears  very  rapidly  with  dilution,  and  which  causes  the 
apparent  rapid  narrowing  of  the  green  band  observed  in  the  spectrogram. 

In  B  the  green  band  also  narrows  considerably,  its  limits  in  the  first 
solution  being  X  4900  and  X  5430,  and  in  the  seventh  solution  X  5050  and 
X  5320.  The  violet  edge  of  the  band  is  shaded  considerably  in  the  first 
three  solutions;  the  transmission  being  incomplete  as  far  as  X  4500.  In 
the  red  this  set  of  solutions  does  not  show  any  absorption  sufficiently 
intense  to  be  recorded  on  the  photographic  plate. 

A  comparison  of  the  fourth  strip  of  B,  Plate  22,  with  the  fourth  strip  of  B, 
Plate  3,  shows  that  the  green  band  on  the  two  has  about  the  same  intensity 
and  width.  The  depth  of  the  two  corresponding  solutions  was  the  same, 
but  the  concentration  of  the  chloride  solution  was  0.329,  while  that  of  the 


I 


SALTS    OF    COBALT.  33 

sulphocyanate  was  only  0.12.  This  would  indicate  for  the  sulphocyanate 
solution  an  absorbing  power  about  2.5  times  as  great  as  for  the  chloride 
solution,  considering  only  the  green  band.  It  is  also  worth  noticing  that 
whereas  the  green  band  in  the  chloride  solutions  remains  practically  con- 
stant for  concentrations  below  0.4  normal,  when  the  conditions  for  Beer's 
law  are  fulfilled,  the  same  band  in  the  sulphocyanate  solutions  continues 
to  narrow  rapidly,  even  where  the  concentration  is  as  small  as  0.05  nor- 
mal. The  percentage  dissociation  of  solutions  of  the  two  salts  having 
equal  concentrations  is  not  very  different  in  amount. 

Cobalt  Sulphocyanate  in  Water — ^Molecules  Constant.    (See  Plate  23.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.17,  1.78,  1.34,  1.01,  0.78,  0.62,  and  0.51;  the  corresponding 
depths  of  absorbing  layer  were  3,  4,  6,  9,  15,  18,  and  24  mm.  For  B  the 
concentrations  were  0.51,  0.40,  0.30,  0.215,  0.165,  and  0.10;  the  depths 
of  cell  were  the  same  as  for  A.  The  exposures  to  the  Nernst  lamp  and  the 
spark  were,  respectively,  1 J  and  3  minutes,  the  width  of  slit  being  0.01  cm. 

The  ultra-violet  absorption  still  narrows  quite  markedly  with  dilution, 
the  limits  of  transmission  for  the  most  concentrated  and  most  dilute  solu- 
tions of  A  being  X  3630  and  X  3430;  the  corresponding  limits  for  B  were 
X  3290  and  X  3220.  There  is,  then,  a  tendency  for  this  band  to  remain  con- 
stant in  width  as  the  dilution  is  increased,  since  the  narrowing  is  very 
much  less  in  B  than  in  A. 

The  green  band  widens  somewhat,  though  less  than  in  the  case  of  some 
of  the  other  cobalt  solutions  studied.  In  A  the  widening  of  this  band 
towards  the  violet  and  the  narrowing  of  the  ultra-violet  absorption  pro- 
duce the  curious  result  that  a  rather  narrow  region  of  transmission  moves 
continuously  towards  the  ultra-violet  with  dilution,  its  width  remaining 
sensibly  constant.  By  varying  the  depth  of  layer  the  transmission  may 
be  made  to  have  almost  any  width  desired. 

The  limit  of  the  violet  edge  of  the  band  in  A  changes  from  X  4350,  in 
the  most  concentrated  solution,  to  X  4220  in  the  most  dilute.  The  red 
edge  is  at  X  6400  in  the  first  solution.  It  moves  rapidly  towards  the  shorter 
wave-lengths  at  first,  reaching  X  5900  in  the  fifth  solution.  From  this 
point  it  moves  gradually  towards  the  red  with  further  dilution.  The  appar- 
ent narrowing  of  this  band  at  first  is  undoubtedly  due  to  the  band  located 
in  the  red,  which  disappears  rapidly  with  dilution.  If  this  band  were 
absent  we  should  find  that  the  green  band  would  widen  continuously, 
though  slowly,  with  decrease  in  concentration  when  molecules  are  kept 
constant. 

In  B  the  limits  of  the  green  band  in  the  most  concentrated  solution 
are  X  4630  and  X  5500,  while  in  the  most  dilute  solution  they  are  X  4580 
and  X  5520,  thus  showing  a  widening  of  about  70  Angstrom  units.  The 
widening  is  somewhat  unsymmetrical,  which  is  to  be  explained,  however, 
by  the  lack  of  uniformity  in  the  sensibility  of  the  Seed  plate  in  the  region 
occupied  by  the  red  edge  of  the  band. 
3 


34  ABSORPTION   SPECTRA   OF    SOLUTIONS. 

A  comparison  of  the  first  strip  of  B  with  the  first  strip  of  Plate  20  A 
shows  that  the  width  of  the  green  band  in  the  two  is  very  nearly  the  same. 
The  depth  of  cell  used  was  3  mm.  in  each  case,  but  the  concentration  of 
the  nitrate  was  2.05  while  that  of  the  sulphocyanate  was  only  0.51,  indi- 
cating an  absorbing  power  for  the  sulphocyanate  nearly  4  times  as  great 
as  for  the  nitrate. 

Cobalt  Acetate  in  Watbb — Beeb's  Law.     (See  Plate  24.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  nearest  the  numbered  scale, 
were  0.86,  0.68,  0.54,  0.43,  0.34,  0.27,  and  0.22;  the  corresponding  depths 
of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the  concen- 
trations were  0.34,  0.25,  0.21,  0.17,  0.14,  0.11,  and  0.09;  the  depths  of 
cell  were  the  same  as  in  A.  The  color  of  the  solutions  changed  from  pur- 
plish to  the  usual  pink  with  increasing  dilution.  The  exposures  to  the 
Nernst  lamp  and  spark  were  1  and  2  minutes  respectively,  the  slit  having 
a  width  of  0.01  cm.  as  usual. 

The  absorption  in  the  ultra-violet  is  slight,  and  decreases  with  decrease 
in  concentration.  The  limits  of  transmission  for  the  most  concentrated 
and  most  dilute  solutions  of  set  A  are  X  2620  and  X  2470,  and  for  set  B 
X  2470  and  A  2390.  Some  irregularities  in  the  intensities  of  the  various 
spark  spectra  are  noticed,  indicating  considerable  fluctuations  in  the 
intensity  of  the  spark  during  the  dififerent  exposures.  In  spite  of  this 
there  can  be  no  doubt  that  the  spectrogram  shows  an  increase  of  transpar- 
ency with  decreasing  concentration. 

The  green  band  narrows  with  dilution,  but  more  and  more  slowly  as 
the  concentration  is  diminished.  In  the  last  three  or  four  solutions  of 
set  B  it  remains  practically  constant.  The  hmits  of  transmission  for  the 
most  concentrated  and  the  most  dilute  solutions  of  set  A  are  X  4500  and 
X  5600,  and  X  4600  and  X  5520,  respectively,  placing  the  center  of  the  band 
at  about  X  5050  throughout.  In  set  B  the  center  is  near  A  5175.  A  part 
of  this  change  in  position  may,  however,  be  accounted  for  by  the  lack 
of  uniformity  in  the  sensibility  curve  for  the  Seed  film.  From  the  red 
edge  of  the  green  band  the  solutions  transmit  freely  light  of  all  wave- 
lengths as  far  as  beyond  X  7400. 

Strip  1  of  A  may  be  compared  with  the  first  strip  of  Plate  11  A,  and 
it  will  be  found  that  the  width  of  the  green  band  in  the  two  is  very  nearly 
the  same.  The  depth  of  the  absorbing  layer  in  each  case  was  6  mm.,  but 
the  concentration  of  the  bromide  solution  was  1.51,  while  that  of  the 
acetate  was  0.86;  thus  showing  that  the  acetate  solution  absorbs  green 
light  much  more  strongly  than  a  solution  of  the  bromide  having  the  same 
concentration.  A  comparison  of  the  seventh  strip  of  A  with  the  seventh 
strip  of  Plate  23  B  shows  that  the  green  bands  in  the  two  agree  almost 
exactly  in  width  and  position.  The  depth  of  the  cell  in  each  case  was 
24  mm.,  while  the  concentrations  of  the  sulphocyanate  and  acetate  were, 
respectively,  0.10  and  0.22.  This  indicates  that  the  sulphocyanate  solu- 
tion has  about  double  the  absorbing  power  of  the  acetate  solution  of  equal 
concentration,  for  green  light. 


SALTS    OF    COBALT.  35 

GENERAL   SUMMARY   OF   RESULTS   WITH    COBALT    SALTS. 

We  shall  consider  the  aqueous  solutions  first,  since  they  are  the  sim- 
plest from  the  standpoint  of  the  absorption  spectra.  All  of  the  solutions 
studied,  with  two  exceptions,  at  room  temperatures  and  with  such  depths 
of  absorbing  layer  as  were  employed,  show  only  two  regions  of  absorption, 
one  in  the  ultra-violet  and  one  in  the  green.  The  exceptions  are  the  con- 
centrated solutions  of  the  sulphocyanate,  and  the  solutions  of  cobalt 
bromide  to  which  large  amounts  of  calcium  bromide  had  been  added; 
both  of  which  show  some  absorption  in  the  red. 

Let  us  first  consider  the  absorption  in  the  ultra-violet.  Solutions  of 
all  the  salts  studied,  except  the  sulphate,  have  a  region  of  so-called  one- 
sided absorption,  which  cuts  off  more  or  less  of  the  ultra-violet  end  of  the 
spectrum,  depending  upon  the  salt  used.  In  all  cases  the  band  narrows 
with  dilution  when  the  conditions  for  Beer's  law  obtain,  but  tends  to  remain 
approximately  of  constant  width  when  molecules  are  kept  constant.  In 
the  bromide  and  nitrate  the  band  is  constant  with  molecules  constant. 
This  indicates  that  the  absorber  which  is  responsible  for  this  band  is  in 
every  case  the  undissociated  molecule,  and  to  account  for  the  deviations 
from  constancy  in  the  band  when  the  number  of  absorbers  is  kept  constant 
we  may  assume  that  the  molecules  in  concentrated  solutions  associate  to 
some  extent,  and  that  their  absorbing  power  is  thereby  increased;  or  we  may 
assume  that  with  increasing  dilution  they  become  more  and  more  hydrated, 
and  that  this  decreases  their  absorbing  power.  A  choice  between  these 
two  explanations  can  not  be  made  without  a  further  study  of  the  subject. 

In  addition  to  the  one-sided  ultra-violet  band,  cobalt  chloride  has  a 
band  at  X  3300,  which  disappears  rapidly  with  dilution  even  when  mole- 
cules remain  constant.  This  band  seems  to  increase  in  intensity  very 
rapidly  with  rise  in  temperature.  We  can  not  very  reasonably  ascribe 
this  band  to  the  undissociated  molecules  as  such,  since  it  not  only  narrows 
rapidly  but  entirely  disappears  when  these  are  kept  constant.  To  explain 
it  we  must  hence  look  to  either  association  or  hydration.  It  may  be  re- 
marked here  that  by  association  we  do  not  mean  simply  a  grouping  together 
of  similar  particles,  but  also  a  grouping  together  of  such  parts  as  mole- 
cules and  ions,  or  aggregates  of  molecules  and  ions,  etc.  The  term  associa- 
tion, therefore,  includes  the  complex  anions  assumed  to  exist  by  Donnan 
and  Bassett.  Both  association  and  hydration  are  known  to  diminish  with 
rise  in  temperature  (except  Donnan  and  Bassett's  complex  anions);  with 
increasing  concentration  at  a  given  temperature  association  is  known  to 
increase  while  hydration  decreases.  In  a  fairly  dilute  solution  the  amount 
of  association  is  perhaps  negligible.  If  we  then  assume  that  the  X  3300 
band  is  due  to  some  aggregate,  this  would  explain  its  disappearance  on 
dilution,  since  this  process  destroys  the  aggregate.  But  raising  the  tempera- 
ture also  destroys  the  aggregate  without,  however,  causing  the  absorption 
band  to  disappear.  The  fact  is  that  it  becomes  more  intense  as  the  tem- 
perature rises.  It  seems,  therefore,  rather  difficult  to  assume  that  it  is  due  to 
aggregates,  at  least  to  aggregates  which  are  not  abnormal  in  their  behavior. 

If  we  assume  that  the  band  is  due  to  some  relatively  simple  hydrate 
the  facts  are  at  once  accounted  for,  since  with  rise  in  temperature  com- 


36  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

plex  hydrates  break  down  into  simpler  ones.  Also  with  increase  in  dilu- 
tion more  and  more  complex  hydrates  are  formed,  which  would  also  cause 
the  band  to  disappear. 

The  green  band  appears  in  all  aqueous  solutions,  although  with  vari- 
ous intensities  and  apparently  with  somewhat  different  positions.  The 
change  in  position  is  inappreciable  in  the  case  of  dilute  solutions,  and 
with  concentrated  solutions  it  depends  entirely  upon  whether  the  band 
widens  symmetrically  or  not.  In  general,  it  widens  perhaps  a  trifle  more 
towards  the  violet,  especially  at  first.  A  very  slight  amount  of  general 
absorption  will  shift  the  apparent  center  of  an  unsymmetrical  band,  and 
this  is  perhaps  the  explanation  of  the  slight  variation  in  the  position  of 
the  center  of  this  band  in  concentrated  solutions  of  different  salts. 

The  intensity  of  the  band  as  indicated  by  its  width  on  the  photo- 
graphic plate  is  more  interesting.  For  if  it  is  due  to  the  cobalt  cation  as 
such,  it  ought  to  have  a  greater  intensity  in  solutions  which  are  strongly 
dissociated  than  in  slightly  dissociated  solutions,  concentration  and  depth 
of  layer  being  constant. 

The  spectrograms  of  this  chapter  show  the  following:  For  solutions  which 
have  a  concentration  of  2  normal  or  more,  the  salts  arranged  in  the  order 
of  increasing  intensity  of  the  green  band  are,  nitrate,  bromide,  chloride, 
sulphocyanate.  For  dilute  solutions  (concentrations  of  about  0.1  or  0.2 
normal)  the  order  is,  bromide,  chloride,  nitrate,  sulphate,  acetate,  sulpho- 
cyanate. Arranged  in  the  order  of  increasing  dissociation  the  salts  would 
be  acetate,  sulphate,  sulphocyanate,  nitrate,  chloride,  bromide,  which  is  just 
the  opposite  of  the  order  of  increasing  intensity  of  the  green  band,  if  we 
leave  out  the  sulphocyanate.  It  is  very  evident,  therefore,  that  something 
besides  the  cobalt  cation  must  play  a  part  in  the  production  of  this  band. 

Plate  21  shows  that  when  the  concentration  of  the  sulphate  is  varied 
from  0.65  to  0.06  the  width  of  the  band  does  not  vary,  provided  the  hght 
is  made  to  pass  through  such  depths  of  the  solution  that  the  product  of 
concentration  and  depth  remains  constant;  it  follows,  therefore,  that  in 
this  case  the  absorption  is  simply  proportional  to  the  number  of  cobalt 
atoms  in  the  solution,  and  independent  of  whether  these  exist  as  ions, 
combined  with  SO^,  as  molecules,  or  as  parts  of  the  various  aggregates 
or  hydrates  that  we  may  assume  to  exist  in  the  solution.  The  same  is 
approximately  true  for  the  nitrate  solutions,  although  in  this  case  there 
is  a  slight  narrowing  of  the  band,  indicating  that  the  absorbing  power  of 
the  various  "absorbers"  is  not  the  same.  In  general,  the  simplest  expla- 
nation of  the  green  band  is  to  assume,  as  we  have  just  done,  that  the  cobalt 
atom,  no  matter  what  it  is  combined  with,  has  the  power  of  absorbing 
green  light,  the  intensity  of  the  absorption  depending,  however,  upon  the 
nature  of  the  combination. 

In  the  red,  aqueous  solutions  show  little  or  no  absorption  unless  they 
are  very  concentrated,  or  are  at  a  high  temperature,  or  have  relatively 
large  amounts  of  such  substances  as  HCl,  CaClj,  or  AICI3,  etc.,  added 
to  them.  Whether  the  absorption  produced  by  these  different  methods 
is  the  same  or  not  for  any  given  salt  can  not  yet  be  answered  definitely. 
We  have  already  pointed  out  (see  page  23)  that  the  absorption  of  a  solu- 


# 

SALTS    OF    COBALT.  37 

tion  of  cobalt  bromide  to  which  a  large  amount  of  calcium  bromide  is  added 
is  similar  but  not  identical  with  that  of  a  solution  of  the  chloride  of  cobalt 
to  which  a  large  amount  of  calcium  chloride  had  been  added.  The  work 
of  Jones  and  Uhler  ("Hydrates  in  Aqueous  Solution")  indicates  that  the 
absorption  of  cobalt  chloride  when  "dehydrated"  with  calcium  chloride, 
aluminium  chloride,  or  calcium  bromide,  is  the  same,  but  that  this  differs 
somewhat  from  the  absorption  of  very  concentrated  solutions  of  cobalt 
chloride  alone.  In  the  latter  case  only  three  of  the  five  bands  were  seen, 
and  these  were  located  nearer  the  red  end  of  the  spectrum;  but  the  "dis- 
placement" was  different  for  different  bands,  amounting  to  170  A.U.  for 
the  least  refrangible,  and  115  A.U.  for  the  most  refrangible. 

The  entire  absence  of  this  red  absorption  in  solutions  of  moderate 
concentration  shows  at  once  that  it  can  not  be  accounted  for  by  the  simple 
theory  of  dissociation,  according  to  which  there  should  be  present  only 
ions  and  molecules.  The  fact  that  the  absorption  does  appear  in  very 
concentrated  solutions  naturally  suggests  that  it  may  be  due  to  aggre- 
gates of  molecules,  but  this  view  is  not  tenable,  since  the  absorption  in- 
creases with  rise  in  temperature.  The  most  reasonable  explanation,  and 
the  one  that  best  fits  the  facts,  is  that  it  is  due  to  some  relatively  simple 
hydrate  of  the  molecule.  The  conditions  which  favor  the  formation  of  a 
"simple"  hydrate  are  high  temperature,  or  great  concentration,  or  the 
addition  of  large  amounts  of  some  dehydrating  agent  to  a  moderately 
concentrated  solution,  and  in  all  of  these  cases  the  absorption  in  the  red 
appears.  That  this  explanation  is  the  correct  one  is  also  made  probable 
by  the  work  of  Russell  on  the  absorption  of  the  various  dry  salts  of  cobalt. 
The  anhydrous  salts  all  show  absorption  in  the  red,  as  do  also  the  simple 
hydrates;  while  hydrates  containing  6  molecules  of  water  exert  no  absorp- 
tion in  this  region  of  the  spectrum. 

In  non-aqueous  solvents  only  the  chloride  and  bromide  have  been 
studied.  The  chloride,  which  in  aqueous  solutions  showed  an  absorption 
band  near  X  3300,  has  in  the  alcoholic  solutions  two  bands,  one  at  X  3100 
and  the  other  at  X  3600.  These  bands  behave  very  much  like  the  A  3300 
band  in  the  aqueous  solution,  disappearing  quite  rapidly  with  dilution. 
They  are  most  hkely  due  to  some  relatively  simple  solvate.  A  study  of 
the  change  of  absorption  with  temperature  will  undoubtedly  throw  some 
light  on  this  point. 

The  green  band  is  present  in  all  the  non-aqueous  solutions  studied, 
although  its  intensity  in  the  acetone  solutions  is  so  small  that  very  deep 
layers  of  solution  were  necessary  in  order  to  see  even  a  trace  of  it.  This 
is  exactly  what  we  should  expect  if  the  view  is  held  that  the  cobalt  atom, 
no  matter  with  what  it  is  associated,  absorbs  green  light  to  some  extent. 
The  intensity  of  the  band  diminishes  as  we  pass  from  solutions  in  methyl 
alcohol  to  those  in  ethyl  alcohol  and  acetone,  but  so  do  the  concentrations; 
hence,  as  in  aqueous  solutions,  the  intensity  of  the  band  may  be  said  to 
be  roughly  proportional  to  the  concentration. 

In  the  red  the  absorption  is  much  more  intense  in  the  non-aqueous 
than  in  aqueous  solutions,  the  intensity  for  equal  concentrations  increas- 
ing very  rapidly  as  we  pass  from  methyl  alcohol  to  ethyl  alcohol  to  acetone. 


38  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

The  structure  of  the  band  dififers  materially  in  the  dififerent  solvents,  as 
has  been  pointed  out  in  the  description  of  the  spectrograms.  With  dilution 
it  narrows  rapidly  in  methyl  alcohol,  slower  in  ethyl  alcohol,  and  remains 
constant  or  nearly  so  in  acetone.  That  these  changes  can  not  be  explained 
by  dissociation  has  already  been  pointed  out  by  Jones  and  Uhler,  who  sug- 
gested that  they  may  be  due  to  solvation.  This  is  altogether  reasonable, 
and  the  behavior  of  the  spectrum  is  exactly  what  we  should  expect  from 
the  conclusions  arrived  at  in  the  discussion  of  aqueous  solutions.  The 
red  absorption  then  was  ascribed  to  "simple"  hydrates,  such  as  contain 
2  or  3  molecules  of  water  or  less.  It  is  not  unlikely  that  "simple"  solvates 
of  cobalt  salts,  in  general,  have  this  property  of  absorbing  red  light. 

We  should  expect  the  power  to  form  solvates  to  be  greater  for  methyl 
alcohol  than  for  ethyl  alcohol,  and  greater  for  the  latter  than  for  acetone. 
Hence,  in  case  of  ethyl  alcohol  and  acetone  at  ordinary  temperature,  all 
the  solvates  formed  are  perhaps  simple  enough  to  exert  powerful  red  ab- 
sorption, while  with  methyl  alcohol  this  is  only  the  case  wdth  concentrated 
solutions  or  at  elevated  temperatures.  The  differences  in  the  structure 
of  the  band  in  the  different  solvents  are,  of  course,  to  be  expected,  since 
the  "absorbers"  are  different. 


CHAPTER  III. 
SALTS  OF  NICKEL. 

Among  the  more  important  investigations  on  the  absorption  spectra 
of  nickel  salts  are  the  following : 

Brewster,*  in  his  early  work  on  absorption,  included  the  nitrate  of 
nickel,  and  Emsmann  ^  also  studied  the  same  salt. 

Vogel  ^  studied  not  only  cobalt  chloride  but  also  the  chloride  of  nickel, 
in  connection  with  its  power  to  absorb  light. 

The  work  of  Soret  *  in  1878  also  had  to  do  with  the  chloride  of  nickel. 

The  splendid  investigations  of  Hartley^  on  absorption  spectra  included 
also  certain  salts  of  nickel. 

The  work  of  Miiller '  in  connection  with  salts  of  nickel  calls  for  special 
comment.  He  tested  Beer's  law  for  certain  salts  of  nickel  and  copper, 
and  found  that  it  holds  for  the  sulphate  and  nitrate  of  nickel.  The  chlo- 
ride and  bromide  of  nickel  showed  deviations  from  the  law.  The  deviations 
from  Beer's  law  he  thinks  are  to  be  explained  on  the  basis  of  dissociation. 

In  a  subsequent  paper  Miiller '  tests  the  above  suggestion,  and  comes 
to  the  conclusion  that  dissociation  alone  can  not  account  for  all  the  devia- 
tions from  Beer's  law.  If  the  law  does  not  hold,  rise  in  temperature  would 
produce  a  change  in  the  absorption,  and  rise  in  temperature  would  be 
expected  to  produce  a  result  similar  to  increase  in  concentration. 

The  fact  is  that  rise  in  temperature  produces  a  different  effect  on  absorp- 
tion from  increase  in  concentration,  which  shows  that  more  than  one  factor 
must  be  taken  into  account  in  dealing  with  the  causes  of  the  deviation 
from  Beer's  law. 

Muller  thinks  that  both  hydration  and  molecular  complexes  come  into 
play. 

Nickel  Chloride  in  Water — Beer's  Law.     (See  Plate  25.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.66,  2.00,  1.33,  0.89,  0.61,  0.44,  and  0.33,  the  corresponding 
depths  of  layer  being  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the  concentra- 
tions were  0.44,  0.33,  0.22,  0.15,  0.101,  0.073,  and  0.055;  the  depths  of 
layer  were  the  same  as  for  A. 

The  solutions  were  gi-een;  the  more  dilute  ones  tending  towards  a 
light  yellowish-green.  The  exposures  to  the  Nernst  lamp  and  spark  were 
IJ  and  3  minutes,  respectively,  the  slit  having  a  width  of  0.01  cm. 

»  Phil.  Mag.  (4),  24,  441  (1862). 

» Pogg.  Ann.  Erganzb.,  6,  334  (1876). 

»  Ber.  d.  deutsch.  chem.  Gesell.,  8,  1533  (1875). 

•  Archiv.  d.  Sci.  Phys.  et  Nat.,  61,  322  (1878). 

•  Trans.  Roy.  Dub.  Soc.  (2),  7,  253  (1900).    Journ.  Chem.  Soc.,  83,  221  (1903). 

•  Ann.  d.  Phys.,  12,  767  (1903). 
'  Ibid.,  21,  515  (1906). 

39 


40  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

A  shows  three  regions  of  absorption,  one  in  the  extreme  ultra-violet, 
a  band  in  the  end  of  the  visible  violet,  and  a  band  cutting  off  the  extreme 
red.  Besides  this  there  is  a  rather  strong  general  absorption  in  the  entire 
ultra-violet,  beyond  the  absorption  band  in  the  end  of  the  violet. 

The  extreme  ultra-violet  absorption  perhaps  narrows  slightly  with 
dilution,  although  from  the  second  strip  to  the  seventh  the  limit  of  trans- 
mission seems  to  remain  almost  fixed  at  k  2550.  The  first  solution,  how- 
ever, shows  much  more  absorption  in  the  whole  ultra-violet  region,  includ- 
ing the  band  at  ;i  3960. 

The  band  at  A  3960  narrows  from  the  first  to  the  third  strips  (counting 
from  the  scale)  and  then  remains  of  constant  width,  the  limits  of  trans- 
mission being  approximately  X  3700  and  X  4230.  The  scale  in  the  repro- 
duction is  shifted  towards  the  red  by  nearly  50  A.U.,  due  to  the  fact  that 
in  making  the  prints  this  was  adjusted  with  reference  to  the  narrow  com- 
parison strip  seen  at  the  top  of  the  spectrogram.  This  strip  is  displaced,  as 
may  be  seen  by  comparing  the  spark  lines  in  the  ultra-violet.  The  scale 
for  the  red  end  of  the  plate  is,  however,  correctly  placed,  so  there  is  a  shght 
discrepancy  at  the  point  where  the  two  prints  were  joined  together. 

The  absorption  in  the  red  shades  off  very  gradually  through  a  range 
of  wave-lengths  of  about  1000  A.U.,  being  quite  noticeable  on  the  nega- 
tive at  X  6100  in  the  strip  corresponding  to  the  most  concentrated  solution, 
and  at  X  6200  in  the  strip  corresponding  to  the  most  dilute  solution.  The 
limits  of  transmission  for  the  two  strips  are  at  X  7150  and  X  7250,  respec- 
tively. These  measurements  indicate  a  slight  narrowing  of  the  absorp- 
tion band  with  dilution,  but  it  must  be  remembered  that  photographic 
registering  of  the  spectra  is  not  the  best  method  for  studying  such  very 
hazy  absorption  bands,  since  a  very  slight  change  in  the  length  of  expos- 
ure, or  intensity  of  the  source  of  light  used,  may  apparently  shift  the  band 
very  markedly.  The  only  satisfactory  method  for  studying  such  cases  of 
hazy  absorption  is  a  spectrophotometric  determination  of  the  absorption 
coeflBcient  for  a  number  of  wave-lengths  in  the  region. 

In  B  the  absorption  in  the  extreme  ultra-violet  has  disappeared,  the 
last  lines  in  the  spark  showing  as  well  in  the  strips  taken  through  the  solu- 
tions as  in  the  narrow  comparison  strip  with  nothing  but  air  in  the  path 
of  the  beam  of  light. 

The  band  at  X  3960  has  become  faint,  but  still  shows  distinctly  on  the 
negative.  It  remains  unchanged  in  intensity  in  the  seven  strips  on  the 
spectrogram. 

The  absorption  in  the  red,  although  present  as  shown  by  the  green 
color  of  the  solutions  in  their  bottles,  was  of  too  diffuse  a  character  to  be 
registered  on  the  photographic  plate,  which,  hence,  shows  complete  trans- 
mission to  beyond  X  7400.  On  the  whole,  except  for  the  most  concentrated 
solution.  Beer's  law  seems  to  hold  quite  accurately  for  nickel  chloride. 

Nickel  Chloride  in  Water — Ions  Constant.    (See  Plate  47  A.) 
The  concentrations,  beginning  with  the  solution  corresponding  to  the 
strip  nearest  the  numbered  scale,  were  2.66,  0.93,  0.51,  0.305,  0.200,  0.135, 
0.095;  the  depths  of  cell  were  3,  4,  6,  9,  13,  18,  and  24  mm.    The  exposures 


SALTS    OF    NICKEL.  41 

were  1  and  3  minutes  for  the  Nernst  lamp  and  spark,  respectively,  the 
width  of  the  slit  being  0.01  cm. 

The  spectrogram  shows  exactly  what  might  be  expected  from  a  study 
of  Plate  25.  The  extreme  ultra-violet  absorption  disappears  rapidly,  as 
does  the  absorption  in  the  red.  The  band  at  X  3960,  very  wide  at  first, 
narrows  rapidly,  becoming  very  faint  in  the  most  dilute  solution. 

This  plate,  together  with  the  similar  one  for  cobalt  chloride  (Plate 
39  B),  shows  at  a  glance  what  an  insignificant  r61e  ions  play  in  producing 
the  absorption  of  solutions  of  cobalt  and  nickel  salts. 

Nickel  Chloride  in  Water — Molecules  Constant.     (See  Plate  26.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.66,  2.18,  1.63,  1.22,  0.935,  0.750,  and  0.610;  the  correspond- 
ing depths  of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.  For  set 
B  the  concentrations  were  0.800,  0.660,  0.493,  0.365,  0.280,  0.220,  and 
0.160;  the  depths  of  the  cell  were  the  same  as  for  set  A.  The  exposures 
were  1  and  3  minutes,  respectively,  for  the  Nernst  lamp  and  spark,  the 
slit  having  a  width  of  0.01  cm. 

The  general  ultra-violet  absorption  beyond  the  X  3960  band  seems  to 
remain  nearly  constant  in  the  five  most  concentrated  solutions  of  set  A, 
then  increases  markedly  in  the  sixth  and  seventh.  The  A  3960  band  nar- 
rows slightly  from  the  first  to  the  second  strip  (counting  from  the  scale), 
then  begins  to  widen,  slowly  at  first,  but  more  rapidly  from  the  fifth  to 
the  seventh  solutions. 

In  B  the  general  ultra-violet  absorption  may  still  be  seen,  and  it  in- 
creases with  dilution.  The  X  3960  band  also  widens  regularly  with  dilution. 
In  the  red  the  absorption  increases  regularly  with  dilution  in  both  A  and  B. 

Nickel  Chloride  in  Water  with  Calcium  and  Aluminium  Chlorides. 

(See  Plate  27.) 

The  concentration  of  nickel  chloride  throughout  was  constant  and 
equal  to  0.372.  The  concentrations  of  calcium  chloride,  beginning  with 
the  solution  whose  spectrum  is  adjacent  to  the  numbered  scale,  were  3.97, 
3.40,  2.85,  2.30,  1.75,  1.20,  0.64,  and  0.00.  The  corresponding  concentra- 
tions of  aluminium  chloride  were  2.61,  2.25,  1.88,  1.52,  1.16,  0.79,  0.43, 
and  0.00. 

A  is  the  spectrogram  made  with  the  solutions  containing  calcium 
chloride,  while  B  was  made  with  those  containing  the  aluminium  chloride. 
The  two  spectrograms  may  conveniently  be  described  together,  as  this  will 
facilitate  comparisons.  The  strips  will  be  referred  to  as  first,  second, 
third,  etc.,  the  number  giving  the  position  of  the  strip  with  reference  to 
the  numbered  scale. 

In  general  it  may  be  stated  that  the  absorption  in  the  violet  and 
ultra-violet  is  somewhat  greater  for  the  solutions  containing  aluminium 
chloride  than  for  the  corresponding  ones  containing  calcium  chloride. 
This  is  partly  due  to  the  greater  absorption  of  the  aluminium  salt  per 
se  for  ultra-violet  light.  (See  "Hydrates  in  Aqueous  Solution,"  Plate 
11  A.)     The  first  strip  of  A  shows  that  transmission  in  the  violet  ceases 


42  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

at  }.  4320,  and  that  there  is  a  shght  return  to  transparency  at  ^  3000  to 
X  3100,  then  complete  absorption  to  X  2200  or  the  end  of  the  spectrum. 
The  k  3960  band  narrows  regularly  with  curved  edges  as  the  concentrations 
of  the  calcium  salt  decrease,  its  limits  in  the  eighth  strip  being  yl  4150  and 
k  3750.     The  increase  of  transparency  beyond  the  band  is  also  very  marked. 

The  first  strip  of  B  shows  that  transmission  in  the  violet  ceases  at 
X  4370,  and  that  there  is  no  return  to  transparency  in  the  ultra-violet. 
The  red  edge  of  the  X  3960  band  moves  towards  the  region  of  shorter  wave- 
length, rapidly  in  the  first  four  strips,  then  more  slowly  till  the  eighth 
strip  is  reached,  where  the  limit  of  transmission  is  X  4150  as  in  A.  No 
transmission  in  the  ultra-violet  region  beyond  the  band  is  visible  in  the 
first  three  strips.  In  the  fourth  strip  there  is  a  slight  amount  of  transmis- 
sion from  X  2800  to  X  3400,  which  increases  rapidly  as  the  amount  of  alu- 
minium chloride  is  further  diminished. 

The  red  end  of  A  shows  that,  although  the  limit  of  transmission  moves 
slightly  towards  the  region  of  longer  wave-lengths  with  decrease  in  the 
amount  of  calcium  chloride,  the  absorption  in  the  region  X  6000  to  X  6500 
increases  rapidly,  thus  showing  that  on  the  whole  the  absorption  in  the 
red  is  decreased  very  much  by  adding  the  calcium  salt. 

With  the  addition  of  aluminium  chloride  the  absorption  in  the  region 
X  6000  to  X  6500  decreases  slightly,  while  in  the  region  X  6800  to  X  7100  it 
increases  considerably;  on  the  whole,  therefore,  the  red  absorption  is  per- 
haps somewhat  increased. 

The  first  three  solutions  show  three  rather  narrow  absorption  bands, 
whose  wave-lengths  are  X  6110,  X  6250,  and  X  6440.  Of  these  the  first  and 
last  are  rather  faint,  the  one  at  X  6250  fairly  intense.  They  could  not  be 
seen  in  a  layer  of  the  mother-solution  of  aluminium  chloride  20  cm.  deep, 
and  hence  are  not  to  be  ascribed  to  the  aluminium  salt.  A  careful  exami- 
nation of  the  first  strip  on  the  negative  for  A  reveals  faint  traces  of  the 
three  bands,  having  here  about  the  same  intensity  as  they  have  in  the 
second  strip  of  B.  The  bands  are  hence  to  be  ascribed  to  the  nickel  salt. 
A  more  concentrated  solution  of  the  nickel  salt  with  large  quantities  of 
the  dehydrating  agents  would  undoubtedly  have  brought  out  these  bands 
very  much  better. 

Nickel  Sulphate  in  Wateb — Beer's  Law.     (See  Plate  28.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.1,  1.7,  1.32,  1.05,  0.80,  0.65,  and  0.53;  the  corresponding 
depths  of  absorbing  layer  being  6,  7.5,  9.5,  12,  15,  19,  and  24  mm.  The 
concentrations  for  set  B  were  0.80,  0.64,  0.50,  0.40,  0.32,  0.25,  and  0.20; 
the  depths  of  cell  were  the  same  as  in  A.  The  exposures  to  the  Nernst 
lamp  and  spark  lasted  1  and  2  minutes,  respectively,  the  slit  having  a 
width  of  0.01  cm. 

The  spectrogram  shows  that  the  solutions  were  quite  transparent  in 
the  ultra-violet.  That  there  is  a  slight  amount  of  general  absorption 
beyond  the  X  3960  band  may,  however,  be  inferred  from  the  fact  that 
although  the  exposure  for  the  narrow  comparison  strip  was  only  about 


SALTS    OF    NICKEL.  48 

one-fourth  as  long  as  that  for  the  spark  spectrum  in  the  strips,  the  former 
has  about  the  same  intensity  as  the  latter.  Also  the  extreme  ultra-violet 
lines  shown  by  the  comparison  strip  do  not  appear  in  the  spark  spec- 
trum transmitted  through  the  solutions;  which  indicates  the  presence  of 
a  band  in  the  region  below  X  2350. 

The  X  3960  band  has  exactly  the  same  width  in  all  the  strips,  corre- 
sponding to  the  solutions  of  either  A  or  B,  showing  that  Beer's  law  holds 
exactly.  The  limits  of  transmission  in  A  are  k  3600  and  k  4320,  and  in  B 
X  3700  and  X  4250,  which  gives  the  center  of  the  band  in  A  at  A  3960  and 
in  B  at  A  3975. 

In  the  red  A  shows  complete  absorption  beyond  X  6400,  and  very 
strong  shading  from  X  6400  to  X  5900.  Indications  are  that  the  absorption 
in  this  region  decreases  very  slightly  with  dilution.  B  shows  faint  trans- 
mission to  X  7400,  with  rather  strong  shading  from  X  6100.  No  appre- 
ciable change  in  the  absorption  with  dilution  can  be  noted  from  the  spec- 
trogram. A  comparison  of  the  seventh  strip  of  B  with  the  seventh  strip 
of  A,  Plate  25,  shows  that  the  width  of  the  X  3960  band,  as  shown  by  the 
two,  is  almost  exactly  the  same.  The  depth  of  cell  in  both  cases  was  24 
mm.,  but  the  concentration  of  the  chloride  solution  was  0.33,  while  that 
of  the  sulphate  was  only  0.20.  This  indicates  a  greater  absorbing  power 
for  the  sulphate  in  this  region  of  the  spectrum.  A  comparison  of  the 
red  ends  of  the  same  strips  shows  that  the  sulphate  solution  absorbs  the 
red  much  more  intensely  than  the  chloride. 

Nickel  Acetate  in  Water — Beer's  Law.    (See  Plate  29.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.50,  0.40,  0.31,  0.25,  0.20,  0.15,  and  0.13;  the  corresponding 
depths  of  cell  were  6,  7.5,  9.5,  12,  15,  19,  and  24  mm.  For  B  the  con- 
centrations were  0.20,  0.16,  0.13,  0.10,  0.08,  0.06,  and  0.05;  the  depths  of 
cell  were  the  same  as  in  A.  The  exposures  to  the  Nernst  lamp  and  spark 
lasted  for  1  and  2  minutes,  respectively,  the  slit  having  a  width  of  0.01  cm. 

There  is  evidence  of  a  band  in  the  extreme  ultra-violet,  the  Hmit  of 
transmission  in  A  being  X  2400.  The  general  absorption  in  the  region 
between  this  and  the  X  3960  band  is  not  very  strong,  due  to  the  compara- 
tively small  concentration  of  the  solutions  used. 

The  X  3960  band  narrows  somewhat  with  dilution  in  A,  the  fifth,  sixth, 
and  seventh  strips  showing  sUght  transmission  even  at  its  center.  In  B 
the  band  has  become  faint,  though  still  showing  distinctly.  It  remains 
of  sensibly  the  same  intensity  with  dilution.  In  the  red  A  shows  some 
transmission  as  far  as  X  7400,  with  shading  from  about  X  6600.  In  B 
the  red  absorption  was  too  faint  and  diffuse  to  be  registered  on  the 
photographic  plate. 

A  comparison  of  the  seventh  strip  of  A  with  the  seventh  strip  of  Plate 
26  B  shows  that  although  the  absorption  of  the  acetate  is  somewhat 
greater  it  is  not  very  much  so.  The  depth  of  absorbing  layer  used  in  mak- 
ing the  two  strips  was  the  same,  namely  24  mm.,  but  the  concentration  of 
the  chloride  was  0.16  while  that  of  the  acetate  was  only  0.13.    Hence,  the 


44  ABSORPTION    SPECTRA    OF   SOLUTIONS. 

absorbing  power  of  the  acetate  solution  is  greater  than  that  of  the  chloride. 
A  direct  comparison  of  the  absorption  of  the  acetate  and  sulphate  solu- 
tions is  not  possible  from  the  spectrograms,  since  the  concentrations  dif- 
fered too  much;  but  the  indications  both  from  the  comparison  with  the 
chloride  and  from  visual  observations  in  the  red  are  that  the  two  absorb 
about  the  same  if  concentration  and  depth  of  layer  are  equal. 

The  absorption  bands  of  nickel  salts  seem  to  be  very  similar  in  their 
behavior  to  the  green  band  of  cobalt.  In  our  study  of  that  band  we  came  to 
the  conclusion  that  the  absorbing  power  for  green  light  is  a  property  of  the 
cobalt  atom,  which  is  only  slightly  affected  by  its  immediate  surroundings. 
Similarly,  it  appears  from  our  study  of  nickel  salts  that  the  absorption 
shown  by  them  is  a  property  of  the  nickel  atom,  and  there  are  only  a  few 
hints  that  it  is  changed  very  much  by  the  immediate  surroundings. 

One  of  these  is  the  marked  widening  of  the  X  3960  band  in  nickel  chloride 
as  we  approach  a  saturated  solution.  Others  are  the  widening  of  the  same 
band  when  large  quantities  of  calcium  or  aluminium  chlorides  are  added, 
and  the  appearance  of  the  narrower  bands  in  the  orange  and  red,  together 
with  the  change  in  the  general  absorption  there  under  the  same  conditions. 
These  point  to  the  fact  that  the  simplest  hydrates  have  a  somewhat  dif- 
ferent absorption  from  the  more  complex  ones,  all  of  which  (if  there  are 
several)  seem  to  have  about  the  same  action  on  hght.  More  definite  con- 
clusions on  this  subject  must  be  deferred  until  the  investigation  shall  have 
been  extended  to  more  compounds  and  under  more  varied  conditions. 


CHAPTER  IV. 

SALTS   OF   COPPER. 

A  number  of  investigators  have  included  salts  of  copper  among  those 
whose  absorption  spectra  they  have  studied.  The  results  obtained  with 
copper  salts  are,  however,  neither  as  interesting  nor  apparently  as  impor- 
tant as  those  furnished,  for  example,  by  cobalt. 

Hartley  ^  in  his  elaborate  investigations  on  absorption  spectra  included 
the  chloride  and  bromide  of  copper;  and  Miiller,^  in  his  discussion  of  the 
deviations  from  Beer's  law,  dealt  with  the  salts  of  both  copper  and  nickel. 
Hartley  explained  the  color  changes  in  the  case  of  copper  chloride  on 
addition  of  water  as  due  to  the  formation  of  the  compounds  CuCljjHjO 
and  CuCl2,2H20  from  the  compound  CuClj. 

Donnan  and  Bassett,'  in  their  interesting  paper  in  which  they  develop 
the  conception  of  complex  ions  as  the  cause  of  certain  color  changes  in 
solution,  also  include  cupric  chloride. 

Knoblauch  *  also  studied  the  absorption  spectra  of  copper  sulphate. 

We  have  included  in  our  work  certain  of  the  salts  of  copper,  which 
seemed  to  be  most  promising. 

Copper  Chloride  in  Water — Beer's  Law.     (See  Plate  30.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  4.5,  3.37,  2.25,  1.50,  1.038,  0.750,  and  0.562;  the  corresponding  depths 
of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the  concentra- 
tions were  1.5,  1.12,  0.75,  0.50,  0.37,  0.25,  and  0.19;  the  depths  of  cell  were 
the  same  as  in  A.  The  concentrated  solutions  as  viewed  in  their  bottles 
were  green.  With  dilution  the  color  changed  through  greenish-blue  to  a 
rather  light-blue.  The  exposures  to  the  Nernst  lamp  and  spark  were, 
respectively,  1^  and  3  minutes,  the  sHt  having  a  width  of  0.01  cm. 

The  spectrogram  shows  two  regions  of  absorption,  one  in  the  blue, 
violet,  and  ultra-violet,  and  the  other  in  the  red.  The  two  are  evidently 
of  quite  different  character,  since  the  former  narrows  very  rapidly  with 
decrease  in  concentration,  the  other  only  slightly. 

In  A  the  first  strip  shows  that  transmission  ends  at  X  4750,  while  for 
the  seventh  strip  the  limit  is  at  X  3750.  The  change  in  absorption  is  most 
rapid  from  the  second  to  the  fifth  strips,  giving  the  edge  of  the  band  the 
form  of  a  compound  curve.  In  B  the  band  narrows  most  rapidly  from  the 
first  to  the  fourth  strips,  the  corresponding  limits  of  transmission  being 
X  3950  and  X  3400.    In  the  seventh  strip  transmission  ceases  at  X  3250. 

The  edge  of  this  ultra-violet  band  is  fairly  well-defined  throughout, 
differing  in  this  respect  from  that  of  the  red  band,  which  is  somewhat 
hazy,  although  much  less  so  than  was  the  case  with  the  red  band  of  nickel. 

»  Trans.  Roy.  Dublin  Soc.  (2),  7,  253  (1900).        '  Journ.  Chem.  Soc.,  81,  955  (1902). 
»  Ann.  d.  Phys-,  12,  767  (1903).  *  Wied.  Ann.,  43,  738  (1891). 

45 


46  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

In  the  red  the  first  strip  of  A  shows  complete  absorption  from  X  5900 
to  the  end  of  the  spectrum.  The  seventh  strip  shows  transmission  as  far 
as  X  6150.  The  corresponding  readings  for  the  first  and  seventh  strips  of 
B  are  X  6450  and  X  6675.  The  edge  is,  however,  very  indefinitely  defined 
in  B,  the  shading  being  considerable. 

It  appears,  therefore,  that  the  red  band  also  narrows  with  dilution, 
although  much  less  than  the  ultra-violet  one.  It  also  narrows  more  rapidly 
at  first,  giving  the  edge  of  the  band  a  curved  form,  concave  towards  shorter 
wave-lengths.  At  great  dilutions  and  correspondingly  deep  layers  of  solu- 
tion, the  edge  of  the  band  would  in  all  probability  be  straight,  and  per- 
pendicular to  the  length  of  the  strips. 

CoppEB  Chlobidk  in  Water — ^Molecules  Constant.    (See  Plate  31.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  4.53, 3.59,  2.63,  1.97,  1.50,  1.19,  and  0.97;  the  corresponding  depths  of 
absorbing  layer  were  3, 4, 6,  9, 13, 18,  and  24  mm.  For  B  the  concentrations 
were  1.50, 1.22, 0.91, 0.68, 0.52, 0.415,  and  0.335;  the  depths  of  cell  were  the 
same  as  for  A.  The  exposures  to  the  light  of  the  Nernst  lamp  and  spark 
lasted,  respectively,  1^  and  3  minutes,  the  width  of  the  slit  being  0.01  cm. 

In  this  spectrogram  we  find  that  the  absorption  band  in  the  ultra- 
violet still  narrows  rapidly,  while  that  in  the  red  shows  a  tendency  to 
widen  with  dilution.  The  limits  of  transmission  shown  by  the  first  and 
seventh  strips  of  A  are  X  4750  and  X  4100,  with  the  edge  showing  a  com- 
pound curve  similar  to  the  one  in  Plate  30  A,  but  with  less  curvature. 
For  the  first  and  seventh  strips  of  B  the  limits  are  X  3990  and  X  3500,  the 
edge  forming  a  curved  line  convex  towards  the  region  of  short  wave- 
lengths. In  the  red,  the  first  strip  of  A  gives  the  limit  of  transmission  as 
X  6050.  In  the  second  and  third  strips  the  limit  is  a  little  farther  up  in  the 
red,  being  near  X  6075.  In  the  fourth  strip  it  is  again  at  X  6050,  from  which 
it  moves  gradually  towards  shorter  wave-lengths  until  the  seventh  strip  is 
reached,  where  it  is  at  X  5975.  The  edge  of  the  band  is  hence  curved,  with 
the  convex  side  towards  the  longer  wave-lengths. 

In  B  the  band  widens  continuously  with  decreasing  concentration, 
the  limit  of  transmission  for  the  solution  pertaining  to  the  first  strip  being 
X  6500,  while  the  seventh  strip  shows  complete  absorption  at  X  6400.  The 
edge  is  not  sharply  defined,  the  shading  extending  as  far  as  X  5000  with 
considerable  intensity.  From  X  6000  to  X  5950  the  blackening  of  the  nega- 
tive increases  very  rapidly,  and  this  position  of  most  rapid  increase  in 
transmission  seems  to  be  sensibly  the  same  for  all  the  solutions  of  the  one 
series.  This  is  also  nearly  the  position  of  the  limit  of  transmission  for  the 
concentrated  solutions  used  in  A,  and  also  in  Plate  30  A.  Hence,  it  seems 
likely  that  this  is  the  real  limit  of  the  absorption  band. 

Copper  Chloride  in  Methyl  Alcohol — Beer's  Law.    (See  Plate  32.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.744,  0.595,  0.469,  0.372,  0.297,  0.233,  and  0.186;   the  corre- 


SALTS    OF '^COPPER.  47 

spending  depths  of  absorbing  layer  were  6,  7.5,  9.5,  12,  15,  19,  and  24  mm. 
For  B  the  concentrations  were  0.233,  0.186,  0.147,  0.116,  0.093,  0.073,  and 
0.058;  the  depths  of  absorbing  layer  were  the  same  as  in  set  A. 

The  solutions  were  all  green,  the  intensity  of  the  color  only  chang- 
ing with  dilution.  Exposures  to  the  light  of  the  Nernst  lamp  and  spark 
lasted  for  1^  and  3  minutes,  respectively,  the  sHt  having  the  usual  width 
of  0.01  cm. 

As  in  the  aqueous  solutions  we  have  two  regions  of  absorption,  one  in 
the  blue,  violet,  and  ultra-violet,  the  other  in  the  red.  Both  regions  con- 
tract somewhat  on  decreasing  the  concentration  of  the  solutions,  but  the 
one  in  the  violet  region  contracts  much  more  than  the  one  in  the  red. 

The  limit  of  transmission  for  the  first  strip  in  A  is  at  A  4800,  while  for 
the  seventh  strip  it  is  at  X  4630.  The  edge  of  the  band  forms  a  line  which 
is  slightly  curved  at  first,  the  concave  side  being  towards  the  violet.  From 
the  third  to  the  seventh  strips  the  edge  forms  a  line  which  is  sensibly 
straight.  In  B  the  limit  of  transmission  for  the  first  strip  is  X  4420,  while 
for  the  seventh  it  is  X  4200,  the  edge  forming  a  line  which  is  very  nearly 
straight.  From  the  fourth  to  the  seventh  a  slight  curvature  may  be  noted, 
the  convex  side  turning  towards  the  violet.  On  the  whole  this  band  be- 
haves exactly  like  it  does  in  aqueous  solution,  the  only  difference  being 
the  greater  deviation  from  Beer's  law  in  that  case. 

In  the  red,  the  first  strip  in  A  shows  transmission  to  X  6500,  with  shad- 
ing from  X  6050;  the  seventh  strip,  transmission  to  X  6580,  shading  from 
X  6100.  The  first  strip  in  B  shows  transmission  to  X  6950,  shading  percep- 
tibly from  X  6550;  while  the  seventh  strip  shows  transmission  as  far  as 
X  7020,  with  shading  from  X  6600.  In  both  sets,  therefore,  the  band  narrows 
slightly  with  dilution,  and  in  B  quite  uniformly  with  the  decrease  of  con- 
centration. In  A,  however,  the  narrowing  is  considerably  more  rapid  at 
first.  Here,  again,  we  find  that  the  band  behaves  in  a  manner  very  similar 
to  what  we  found  in  the  aqueous  solution,  only  the  change  is  somewhat 
slower.  The  decrease  in  concentration  from  strip  to  strip,  here,  is,  however, 
only  about  half  what  it  was  in  the  aqueous  solution;  and  taking  this  into 
consideration  the  difference  is  not  as  great  as  it  seems  at  first  glance. 

Copper  Chlobide  in  Ethtl  Alcohol — Beeb's  Law.     (See  Plate  33.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.744,  0.595,  0.469,  0.372,  0.297,  0.233,  and  0.186;  the  depths 
of  absorbing  layer  were  6,  7.5,  9.5,  12,  15,  19,  and  24  mm.  For  set  B  the 
concentrations  were  0.233,  0.186,  0.147,  0.116,  0.093,  0.073,  and  0.058; 
the  depths  of  cell  were  the  same  as  for  set  A. 

All  the  solutions  were  green  as  seen  in  their  bottles,  the  color  being 
somewhat  more  intense  than  was  the  case  with  the  methyl  alcohol  solutions. 

The  exposures  were  made  only  to  the  Nernst  lamp,  since  it  was  shown 
that  all  the  solutions  were  opaque  beyond  the  visible  spectrum.  The 
exposure  lasted  for  1^  minutes,  the  width  of  the  slit  being  as  usual  0.01  cm. 

We  have  the  same  bands  as  in  the  methyl  alcohol  solutions,  with  the 
difference  that  here  they  are  somewhat  wider. 


48  ABSORPTION    SPECTRA   OF   SOLUTIONS. 

The  limits  of  transmission  for  the  first  and  seventh  solutions  in  A  are, 
respectively,  X  5250  and  A  5080,  the  edge  forming  a  line  which  curves 
slightly  from  the  first  to  the  fourth  strips,  then  becomes  straight.  The 
corresponding  limits  for  B  are  A  4750  and  X  4600,  the  edge  forming  a  line 
which  is  straight  from  the  first  to  the  fifth  solutions,  then  curves  slightly 
towards  the  seventh,  the  convex  side  being  towards  the  shorter  wave- 
lengths. It  will  be  noticed  that  the  edge  of  the  absorption  band  here  is 
very  similar  to  what  we  found  in  the  methyl  alcohol  solutions,  the  only 
difference  being  that  here  it  is  located  about  400  Angstrom  units  nearer 
the  red  end  of  the  spectrum. 

The  absorption  in  the  red  apparently  obeys  Beer's  law  quite  accurately, 
the  edge  of  the  band  remaining  in  practically  the  same  position  for  the 
seven  solutions  of  a  set.  This  position  for  A  is  at  A  6200,  for  B  at  A  6700. 
The  shading  in  A  is  noticeable  at  X  5900,  in  B  at  A  6400.  Here,  then,  the 
edge  of  the  band  is,  on  the  whole,  nearer  the  violet  end  of  the  spectrum  by 
about  250  Angstrom  units  than  was  the  case  in  the  methyl  alcohol  solutions. 

CopPEB  Chloride  in  Acetone — Beer's  Law.    (See  Plate  34.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
B,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.02,  0.0168,  0.0139,  0.0114,  0.0094,  0.0080,  and  0.0066,  the 
corresponding  depths  of  absorbing  layer  being  8,  9.5,  11.5,  14,  17,  20,  and 
24  mm.  For  set  A  the  concentrations  were  varied  from  0.008  to  0.0027, 
the  depths  of  cell  being  the  same  as  in  B. 

The  most  concentrated  solutions  were  greenish-yellow,  from  which  the 
color  changed  gradually  to  a  pale  yellow  with  dilution. 

The  exposures  were  made  to  the  light  from  the  Nernst  lamp  only,  a 
preliminary  trial  showing  that  the  entire  ultra-violet  region  was  absorbed 
by  a  comparatively  shallow  layer  of  the  most  dilute  solution.  The  time 
of  exposure  was  IJ  minutes;  the  slit  having  a  width  of  0.01  cm.  as  usual. 

There  is  a  region  of  absorption  in  the  violet  which  in  B  first  narrows 
slightly  with  dilution,  then  begins  to  widen,  and  continues  to  do  so  through- 
out the  entire  range  of  concentrations  studied.  The  limit  of  transmission 
for  the  most  concentrated  solution  of  set  A  is  at  A  4130.  In  the  strip  corre- 
sponding to  the  third  solution  it  is  at  A  4100,  from  which  it  moves  regularly 
towards  the  red,  reaching  X  4150  in  the  seventh  strip. 

In  the  first  solution  of  set  A  the  limit  of  transmission  is  at  X  3850.  It 
moves  towards  the  red  quite  regularly  with  dilution,  reaching  X  3950  in 
the  strip  corresponding  to  the  seventh  solution.  A  shows  an  absorption 
band  having  its  center  at  X  4750.  This  band,  which  is  about  200  A.U. 
wide,  remains  of  constant  width  throughout.  In  A,  though  still  visible, 
it  is  rather  faint.    Its  position  is,  however,  exactly  the  same  as  in  B. 

In  the  red,  both  B  and  A  show  transmission  as  far  as  X  7400,  or  to  the 
limit  of  the  sensibility  of  the  plates  used.  The  slight  shading  in  B,  how- 
ever, indicates  some  absorption  in  the  extreme  red,  and  also  points  to  the 
conclusion  that  this  absorption  increases  somewhat  with  dilution. 

This  is  the  first  case  we  have  found  where  the  absorption  increases  with 
dilution  when  the  product  of  concentration  and  depth  of  absorbing  layer 


SALTS    OF    COPPER,  49 

remains  constant,  and  hence  deserves  careful  consideration.  Jones  and 
Uhler  found  that  they  could  not  use  solutions  of  copper  bromide  in  acetone, 
on  account  of  the  chemical  action  which  takes  place  when  the  two  sub- 
stances are  brought  together.  It  is  barely  possible  that  some  slight  chem- 
ical change  was  taking  place  in  these  solutions  of  copper  chloride  in  ace- 
tone, which  might  not  have  been  sufficient  to  produce  any  precipitation, 
and  which  might  yet  have  increased  with  dilution  in  such  a  way  as  to 
produce  the  effect  observed  in  the  absorption  spectrum.  The  only  other 
case  found  in  this  work  where  this  kind  of  a  deviation  from  Beer's  law 
was  observed  was  that  of  ferric  chloride  in  acetone,  and  here  the  chemical 
action  was  very  noticeable  indeed,  the  color  of  the  solution  deepening 
very  markedly  in  the  course  of  a  few  hours.  There  is  hence  a  reasonable 
doubt  that  the  effect  here  observed  is  real,  and  until  this  is  decided  it  is 
better  not  to  draw  any  conclusions  from  the  spectrogram  just  considered. 

Copper  Chlobide  in  Methyl  Alcohol  with  Water.    (See  Plate  35.) 

The  concentration  of  the  copper  salt  was  constant  throughout,  and 
equal  to  0.15  normal.  The  percentages  of  water  in  the  solutions,  beginning 
with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale,  were  0,  4, 8, 
12,  16,  20,  22,  24,  26,  28,  30,  32,  34,  36,  38,  and  40.  The  common  depth  of  ab- 
sorbing layer  was  2.0  cm.  The  exposures  to  the  light  of  the  Nernst  lamp  and 
spark  lasted  1  and  3  minutes,  respectively,  the  width  of  the  slit  being  0.01  cm. 

In  the  first  solution  transmission  in  the  blue  ceases  at  A  4500.  From 
this  the  limit  of  transmission  moves  towards  the  shorter  wave-lengths, 
fairly  regularly  with  increase  in  the  percentage  of  water  in  the  solution. 
It  will  be  noticed  that  the  increments  in  the  percentages  of  water  were 
4  per  cent  from  the  first  to  the  sixth  solutions,  whence  they  were  2  per  cent 
to  the  end  of  the  series.  The  absorption  in  the  region  of  short  wave-lengths 
also  recedes  more  rapidly  from  the  first  to  the  sixth  strips,  then  more 
slowly,  but  with  perfect  regularity,  until  the  last  strip  is  reached,  where 
the  limit  of  transmission  is  X  3750.  The  line  formed  by  the  edge  of  the 
absorption  band  in  the  first  six  strips  is  curved  somewhat,  the  concave 
side  facing  the  region  of  short  wave-lengths.  This  is  what  we  have  always 
found  with  the  more  concentrated  solutions  of  copper  chloride.  From  the 
sixth  to  the  sixteenth  strips  the  line  is  nearly  straight,  the  curvature,  if 
any,  being  in  the  opposite  direction  to  what  it  is  for  the  first  six  strips. 
In  the  red  the  absorption  band  behaves  in  an  unusual  manner,  as  may 
be  seen  from  the  plate,  but  which  is  more  easily  made  out  from  the  nega- 
tives, of  which  the  following  is  a  description. 

The  strip  corresponding  to  the  solution  containing  no  water  shows 
complete  absorption  of  all  wave-lengths  longer  than  X  6500.  The  absorp- 
tion is  very  weak  at  X  6100,  and  quite  weak  even  at  X  6300,  the  absorption 
band  accordingly  showing  a  fairly  well-defined  edge  in  the  neighbor- 
hood of  X  6400.  With  addition  of  water  up  to  12  per  cent  the  absorption 
increases  markedly,  the  limit  of  transmission  for  the  fourth  strip  still 
being  at  X  6500,  but  the  shading  has  increased  very  much,  the  absorption 
being  now  as  great  at  X  6100  as  it  was  at  X  6300  in  the  first  solution.  Some 
absorption  is  evident  as  far  down  as  X  6000.  From  the  fourth  to  the  six- 
4 


60  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

teenth  solutions,  the  percentage  of  water  increased  from  12  to  40  per  cent, 
the  absorption  diminishing  slightly;  the  limit  of  transmission  in  the  six- 
teenth solution  is  at  X  6650.  The  shading,  although  still  noticeable  as 
far  down  as  X  6000,  is  much  weaker  than  in  the  fourth  solution.  The  point 
to  be  specially  noted  is  that  the  greatest  change  takes  place  at  first,  with 
addition  of  water,  the  change  in  the  total  amount  of  absorption  being 
greater  from  the  first  to  the  second  solution  than  from  the  second  to  the 
third,  and  so  on.  The  effect  of  the  water  may  then  be  said  to  be  two- 
fold. First,  it  tends  to  make  the  edge  of  the  band  much  more  hazy,  and 
secondly,  as  more  of  it  is  added,  it  tends  to  narrow  up  the  band  somewhat. 

Copper  Chloride  in  Ethyl  Alcohol  with  Water.    (See  Plate  36.) 

The  concentration  of  copper  chloride  was  constant  throughout  and 
equal  to  0.10  normal.  The  percentages  of  water  in  the  solutions,  begin- 
ning with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  0,  4,  8,  12,  16,  20,  24,  28,  32,  36,  40,  44,  48,  52,  56,  and  60,  the  strips 
being  all  the  same  and  equal  to  4  per  cent. 

With  addition  of  water,  the  color  of  the  solutions  as  seen  in  the  bottles 
changed  from  an  olive-green  to  a  light-blue.  The  common  depth  of  cell 
was  2.0  cm.  Exposures  to  the  light  of  the  Nernst  lamp  and  spark  lasted, 
respectively,  1  and  3  minutes,  the  slit  having  the  usual  width  of  0.01  cm. 

The  limit  of  transmission  in  the  blue  and  violet  region  moves  towards 
shorter  wave-lengths  with  addition  of  water,  more  rapidly  at  first,  then  more 
and  more  slowly  as  the  percentage  of  water  is  increased,  giving  the  edge  of 
the  band  a  curved  form,  the  convex  side  being  towards  the  region  of  shorter 
wave-lengths.  The  limit  of  transmission  for  the  solution  containing  no 
water  is  X  4800,  and  for  the  one  containing  60  per  cent  of  water  it  is  at 
X  3400.  The  solution  containing  40  per  cent  of  water  ceases  to  transmit  at 
X  3570,  whereas  for  the  methyl  alcohol  solution  containing  40  per  cent  of 
water  it  is  at  X  3750.  When  we  consider  that  the  limit  of  transmission  for 
the  solution  in  pure  ethyl  alcohol  is  X  4800,  while  for  that  in  pure  methyl 
alcohol  it  is  at  X  4500,  we  see  how  much  more  rapidly  the  absorption  of  the 
ethyl  alcohol  solution  decreases  with  addition  of  water.  The  difference  is  no 
doubt  to  be  accounted  for  by  the  smaller  concentration  of  the  metallic  salt 
in  the  case  now  under  discussion,  the  change  in  the  absorption  apparently 
being  determined  by  the  ratio  of  the  amount  of  water  to  the  amount  of 
dissolved  salt,  rather  than  by  the  actual  percentage  of  water  in  the  solvent. 

In  the  red  the  absorption  decreases  regularly  with  addition  of  water. 
The  limit  of  transmission  for  the  first  solution  is  at  X  6400,  the  shading 
extending  to  X  6000.  For  the  sixteenth  solution  the  absorption  is  complete 
at  X  6900,  shading  extending  down  to  about  X  6400.  No  trace  of  the  effect 
observed  in  the  case  of  methyl  alcohol  solutions  was  found. 

Coppeb'Chloridb  in  Acetone  with  Water.    (See  Plate  37.) 

The  concentration  of  the  copper  salt  was  constant  throughout  and 
equal  to  0.014  normal.  The  percentages  of  water  in  the  solutions,  begin- 
ning with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  0,  1,  2,  3,  4,  6,  8,  10,  12,  14,  16,  18,  20,  22,  26,  and  30.  The  depth 
of  cell  throughout  was  2.0  cm. 


* 

SALTS    OF    COPPER.  51 

The  exposures  to  the  hght  of  the  Nernst  lamp  and  spark  lasted  1^  and 
3  minutes,  respectively,  the  slight  increase  in  the  length  of  exposure  to  the 
Nernst  lamp  being  occasioned  by  the  fact  that  the  line  voltage  had  dropped 
so  low  that  it  was  no  longer  possible  to  keep  the  current  through  the  lamp 
at  the  usual  value  of  0.8  ampere.  It  was  accordingly  kept  at  0.76  am- 
pere, and  the  time  of  exposure  lengthened  as  indicated.  Before  any  more 
work  was  done,  the  line  voltage  was  permanently  raised  by  an  adjustment 
of  the  step-down  transformers,  so  that  the  current  could  always  be  kept 
at  0.8  ampere. 

The  solution  in  pure  acetone  absorbed  completely  all  wave-lengths 
shorter  than  X  5250.  As  the  percentage  of  water  was  increased  the  absorp- 
tion moved  rapidly  towards  the  violet  at  first,  then  more  slowly,  becoming 
almost  stationary  towards  the  last.  The  second,  third,  and  fourth  strips 
show  the  presence  of  the  absorption  band  at  X  4730.  This,  however,  dis- 
appears very  rapidly  with  addition  of  water.  The  limit  of  transmission 
for  the  fifth  solution,  containing  4  per  cent  of  water,  is  at  ^4500,  while  for 
the  sixteenth,  with  30  per  cent  of  water,  it  is  at  X  4350. 

In  the  red  the  absorption  is  slight.  The  solution  in  pure  acetone,  how- 
ever, shows  considerable  general  absorption  throughout  the  entire  red 
region,  and  also  indicates  the  presence  of  a  band  beyond  X  7000.  This 
band  narrows  with  increase  in  the  percentage  of  water,  no  sign  of  it  being 
visible  in  the  strip  corresponding  to  the  seventh  solution. 

Copper  Bbomide  in  Water — ^Beer's  Law.     (See  Plate  38.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  1.08,  0.72,  0.50,  0.36,  and  0.27,  the  corresponding  depths  of 
absorbing  layer  being  6,  9,  13,  18,  and  24  mm.  For  B  the  concentrations 
were  0.18,  0.12,  0.083,  0.06,  and  0.045,  the  depths  of  cell  being  the  same 
as  for  A.  The  concentration  of  the  mother-solution  of  copper  bromide  was 
2.16,  but  its  color  was  so  deep-brown,  and  there  was  such  an  amount  of 
general  absorption  throughout  the  spectrum,  that  it  was  impossible  to  get 
any  light  of  sufficient  intensity  to  affect  a  photographic  plate  through  a 
layer  of  it  having  a  depth  of  3  mm.  Even  when  diluted  to  1.62  normal 
a  layer  of  4  mm.  in  thickness  absorbed  practically  everything  except  a 
limited  region  in  the  orange-red.  The  solution  whose  concentration  was 
1.08  had  a  dark  olive-green  color,  the  change  in  color  from  1.6  to  1.08 
normal  being  very  rapid  and  striking.  With  decrease  in  concentration 
below  1.08  the  color  changed  gradually  to  a  light-blue. 

The  exposures  to  the  light  of  the  Nernst  lamp  and  spark  lasted,  respec- 
tively, 1^  and  3  minutes,  the  sHt  having  the  usual  width  of  0.01  cm. 

The  bands  in  the  ultra-violet  and  red  both  narrow  considerably  with 
decrease  in  concentration,  the  narrowing  being  very  much  greater  for  the 
band  of  shorter  wave-lengths.  The  most  concentrated  solution  in  A  ab- 
sorbed everything  of  shorter  wave-length  than  X  4550,  while  the  most 
dilute  solution  transmitted  as  far  down  as  X  3900.  For  B  the  correspond- 
ing limits  are  X  3400  and  X  3120.  The  line  formed  by  the  limits  of  trans- 
mission is  visibly  curved,  the  convex  side  turning  towards  the  region  of 


52  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

short  wave-lengths.  In  the  red,  the  limit  of  transmission  for  the  most 
concentrated  solution  of  set  A  is  at  A  6400,  and  for  the  most  dilute  at 
X  6550.    For  B  the  corresponding  figures  are  A  7250  and  X  7330. 

Copper  Bbomide  in  Wateb — ^Molecules  Constant.    (See  Plate  39  A.) 

The  concentrations  of  the  solutions,  beginning  with  the  one  whose 
spectrum  is  adjacent  to  the  numbered  scale,  were  1.50,  1.22,  0.91,  0.68, 
0.52,  0.415,  and  0.335;  the  depths  of  absorbing  layer  were  3,  4,  6,  9,  13, 
18,  and  24  mm.,  respectively. 

No  data  giving  the  dissociation  of  copper  bromide  were  at  hand,  and 
hence  it  was  assumed  to  be  the  same  as  that  of  copper  chloride.  This 
assumption  is  perhaps  not  absolutely  correct,  but  the  change  in  dissocia- 
tion with  change  in  concentration  is  undoubtedly  so  nearly  the  same  that 
the  conditions  of  "molecules  constant"  were  fulfilled  in  the  series  as  given 
to  a  very  high  degree  of  accuracy. 

The  concentrations  and  depths  of  cell  are  exactly  the  same  as  those 
used  in  making  B  of  Plate  31;  hence  the  two  spectrograms  serve  well  for 
comparing  the  absorbing  powers  of  copper  chloride  and  copper  bromide. 

The  limits  of  transmission  for  the  first  and  seventh  solutions  in  the 
region  of  shorter  wave-lengths  are  X  5400  and  X  3990,  respectively,  those 
in  the  red  being  X  6550  and  X  6400.  It  appears,  therefore,  that  the  bromide 
absorbs  more  strongly  in  the  violet  than  does  the  chloride,  while  the  two 
have  about  the  same  absorbing  power  in  the  red.  It  might  be  argued 
that  since  the  copper  bromide  molecule  is  heavier  than  that  of  copper 
chloride,  we  should  expect  both  absorption  bands  of  the  former  to  be 
shifted  towards  the  red;  and  that  taking  this  shift  into  account  the  result 
would  indicate  a  greater  absorbing  power  for  the  bromide  throughout. 
If  this  were  so,  then  we  should  expect  that,  with  decrease  in  concentration, 
the  absorption  bands  of  the  bromide  would  move  towards  the  violet,  as 
referred  to  the  same  bands  for  the  chloride.  The  reason  for  this  is  that 
with  decrease  in  concentration  the  salts  become  more  and  more  strongly 
dissociated,  and,  hence,  in  dilute  solutions  the  spectra  ought  to  resemble 
each  other  more  and  more  closely.  Now,  the  violet  edge  of  the  region 
of  transmission  moves  towards  shorter  wave-lengths  by  about  the  same 
amounts  for  the  two  salts,  when  the  changes  in  concentrations  are  the 
same.  The  red  band  seems  to  move  a  little  more  towards  the  violet  in  the 
bromide  than  in  the  chloride,  from  the  measurements  given;  but  on  super- 
imposing the  two  negatives  the  two  seemed  identical,  except  for  the  strip 
corresponding  to  the  most  concentrated  bromide  solution,  which  shows 
more  absorption,  due  to  the  large  amount  of  general  absorption  of  this 
solution.  As  stated  before,  measurements  on  the  limits  of  transmission  in 
the  case  of  absorption  bands,  having  such  hazy  edges  as  the  one  we  are 
dealing  with  here,  are  liable  to  very  considerable  errors. 

Copper  Bromide  in  Methyl  Alcohol — Beer's  Law.    (See  Plate  40.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.089,  0.071,  0.056,  0.045,  0.036,  0.028,  and  0.022,  the  corre- 


SALTS    OF    COPPER.  53 

spending  depths  of  absorbing  layer  being  6,  7.5,  9.5,  12,  15,  19,  and  24  mm. 
For  B  the  concentrations  were  0.028,  0.022,  0.018,  0.014,  0.011,  0.009,  and 
0.007;  the  depths  of  cell  were  the  same  as  for  A. 

The  more  concentrated  solutions  were  reddish-brown,  from  which  the 
color  changed  on  dilution  to  a  pale  greenish-yellow. 

The  exposure  to  the  Hght  of  the  Nernst  lamp  was  IJ  minutes  with  a 
width  of  slit  of  0.015  cm.  for  A,  and  50  seconds  with  a  slit  width  of  0.012 
cm.  for  B.  No  exposures  to  the  light  from  the  spark  were  made,  as  a 
preliminary  test  showed  that  even  the  most  dilute  solution  was  opaque 
in  the  ultra-violet. 

In  the  region  cf  short  wave-lengths,  the  limits  of  transmission,  corre- 
sponding to  the  most  concentrated  and  most  dilute  solutions  of  A,  are 
X  4800  and  X  4550,  while  for  B  the  corresponding  Hmits  are  at  k  4320  and 
X  4170,  respectively.  The  hne  formed  by  the  edge  of  the  absorption  band 
is  very  nearly  a  straight  line  in  both  cases. 

In  the  red  the  absorption  is  slight.  The  limits  of  transmission  for  the 
most  concentrated  and  most  dilute  solutions  of  A  are  A  7150  and  X  7200, 
respectively.  In  B  the  edge  of  the  band  is  sensibly  straight,  and  at  right 
angles  to  the  length  of  the  spectrum  strips,  its  limit  being  at  X  7350.  This 
is,  however,  so  near  the  limit  of  sensibility  of  the  plates  used  that  there 
was,  perhaps,  transmission  to  some  little  distance  beyond  this.  Comparing 
this  spectrogram  with  that  of  copper  chloride  in  methyl  alcohol  (Plate  32), 
it  is  seen  that  the  ultra-violet  absorption  in  the  two  cases  is  nearly  iden- 
tical, notwithstanding  the  fact  that  the  concentration  of  the  chloride  was 
about  nine  times  as  great  as  that  of  the  bromide.  This  shows  at  once 
the  great  power  of  the  bromide  for  absorbing  light  of  short  wave-lengths, 
when  dissolved  in  methyl  alcohol.  The  absorption  for  red  light  is  perhaps 
not  very  different  for  the  two  salts.  A  comparison  is  not  possible,  owing 
to  the  great  difference  in  concentration. 

W^m.  Copper  Bromide  in  Ethyij  Alcohol — Beer's  Law.     (See  Plate  41.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.0447,  0.036,  0.028,  0.022,  0.018,  0.014,  and  0.011;  the  corre- 
sponding depths  of  absorbing  layer  were  6,  7.5,  9.5,  12,  15,  19,  and  24  mm. 
For  B  the  concentrations  were  0.014,  0.011,  0.0088,  0.0070,  0.0056,  0.0044, 
and  0.0035;  the  depths  of  cell  were  the  same  as  in  A. 

The  more  concentrated  solutions  were  deep  brownish-red,  opaque  in 
layers  of  any  considerable  depth,  from  which  the  color  changed  on  dilu- 
tion to  a  light  clear-yellow. 

The  exposures  to  the  light  of  the  Nernst  lamp  were  1^  minutes  with  a 
slit  0.02  cm.  wide  for  A,  and  50  seconds  with  a  slit  0.012  cm.  wide  for  B. 
No  exposure  to  the  light  of  the  spark  was  made,  since  all  the  solutions 
were  opaque  in  the  ultra-violet. 

Towards  the  ultra-violet  the  first  solution  of  A  transmitted  faintly  as 
far  as  X  5500;  the  seventh  solution  as  far  as  X  5050.  The  corresponding 
limits  for  B  are  X  4500  and  X  4300,  respectively.  The  edge  of  the  band  in 
either  case  forms  very  approximately  a  straight  line.    In  the  red  the  ab- 


54  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

sorption  is  very  slight,  the  limit  of  transmission  as  shown  by  A  being  at 
X  7200  and  }.  7250,  respectively,  for  the  most  concentrated  and  most  dilute 
solution.  The  solutions  of  B  transmitted  perfectly  to  beyond  X  7400; 
hence  no  absorption  is  registered  on  the  photographic  plate. 

Compared  with  the  solutions  in  methyl  alcohol,  these  solutions  of 
copper  bromide  in  ethyl  alcohol  show  very  much  stronger  absorption  in 
the  region  of  shorter  wave-lengths.  In  the  red  the  absorption  in  the  two 
solvents  is  not  very  different,  if  the  differences  in  concentration  are  taken 
into  account  in  making  the  comparison. 

Copper  Bromide  in  Mbthtl  Alcohoij  with  Water.    (See  Plate  42.) 

The  concentration  of  the  copper  salt  was  constant  throughout,  and 
equal  to  0.05  normal.  The  percentages  of  water  in  the  solutions,  beginning 
with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale,  were  0,  4, 
8,  10,  12,  14,  16,  18,  20,  24,  28,  32,  36,  40,  44,  and  50.  The  depth  of  the 
absorbing  layer  was  constant  and  equal  to  2.0  cm. 

The  solution  containing  no  water  was  brown,  and  practically  opaque 
in  deep  layers.  With  addition  of  water  the  color  changed  rapidly  to  yellow- 
ish-green, and  finally  became  bluish-green  in  the  solutions  containing  the 
greatest  amount  of  water. 

The  time  of  exposure  to  the  light  of  the  Nernst  lamp  and  spark  was, 
respectively,  1|  and  3  minutes,  the  slit  having,  as  usual,  a  width  of  0.01  cm. 

The  limits  of  transmission  for  the  first  eight  solutions,  beginning  with 
the  one  containing  no  water,  were  X  5450,  X  4900,  X  4625,  X  4520,  X  4450, 
X  4330,  X  4270,  and  X  4220.  Leaving  out  the  first  two,  where  there  was 
considerable  general  absorption,  the  limits  fall  almost  exactly  on  a  straight 
line.  The  solution  containing  50  per  cent  of  water  transmitted  as  far  down 
as  X  3700. 

In  the  red  the  absorption  band  also  narrows  and  quite  regularly  with 
addition  of  water,  the  bromide  behaving  in  this  respect  quite  differently 
from  the  chloride.  The  extreme  limit  of  transmission  for  the  solution 
containing  no  water  was  X  6850,  there  being  considerable  absorption  from 
X  6600  on.  For  the  solution  containing  the  largest  percentage  of  water  the 
limit  was  X  7250,  with  considerable  shading  from  X  6650  on.  The  edge  of 
the  band  hence  becomes  more  hazy  with  addition  of  water  here  also,  as 
it  did  in  the  case  of  the  chloride. 

A  comparison  of  this  spectrogram  with  the  one  of  the  chloride  in  methyl 
alcohol  (Plate  35)  shows  not  only  that  the  absorption  of  the  bromide  in  the 
ultra-violet  is  stronger,  but  also  that  it  decreases  much  more  rapidly  on 
addition  of  water.  This  is  undoubtedly  due  to  the  much  smaller  concen- 
tration of  the  bromide,  which  would  make  the  ratio  of  water  to  colored 
salt  very  much  greater  than  it  was  for  the  chloride.  In  the  red  the  absorp- 
tion is  about  what  we  should  expect  from  the  concentration  of  the  solutions. 

Copper  Bromide  in  Ethtl  AlcohoIi  with  Water.     (See  Plate  43.) 

The  concentration  of  the  copper  bromide  w^as  constant  throughout, 
and  equal  to  0.06  normal.  The  percentages  of  water  in  the  solutions, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 


SALTS    OF    COPPER.  55 

were  0,  4,  6,  8,  10,  12,  14,  16,  18,  20,  22,  24,  26,  30,  35,  and  40.  The  depth 
of  the  absorbing  layer  was  in  this  case  only  0.4  cm.,  owing  to  the  practical 
opacity  of  the  solutions  containing  the  least  amount  of  water  in  layers  of 
2.0  cm.  or  more. 

The  first  six  solutions  changed  from  a  deep-brown  to  a  greenish-brown. 
From  the  seventh  to  the  sixteenth  the  color  changed  from  a  clear  bluish- 
green  to  a  hght  greenish-blue.  The  exposures  to  the  light  of  the  Nernst 
lamp  and  spark  were,  respectively,  IJ  and  3  minutes  in  length;  the  width 
of  the  sht  was  0.01  cm. 

Leaving  out  the  first  two  solutions,  where  the  general  absorption  was 
so  great  that  very  little  light  was  transmitted,  we  find  for  the  limit  of 
transmission  towards  the  ultra-violet  for  the  third  solution  X  4750.  From 
this  the  limit  moves  gradually  to  the  shorter  wave-lengths,  reaching  X  3450 
in  the  solution  containing  the  greatest  amount  of  water.  The  absorption 
band  here  narrows  much  more  rapidly  with  addition  of  water  than  was  the 
case  with  the  solutions  in  methyl  alcohol.  A  part  of  this  effect  is  no  doubt 
due  to  the  shallower  layer  of  the  solutions  here  used,  but  that  will  hardly 
account  for  all  of  it,  since  the  edge  of  the  band  is  comparatively  sharp, 
showing  that  it  does  not  widen  very  rapidly  with  increase  in  the  depth  of 
the  solution.  It  can  not  be  accounted  for  by  a  difference  in  concentration, 
as  could  be  done  in  the  case  of  the  chloride  solutions,  for  here  the  actual 
concentration  of  the  ethyl  alcohol  solutions  was  greater  than  that  of  the 
solutions  in  methyl  alcohol.  The  effect  is,  hence,  very  probably  due  to 
some  action  of  the  non-aqueous  solvent,  or  possibly  to  some  mutual  action 
of  the  water  and  the  non-aqueous  solvent,  which  would  be  different  for 
the  two  alcohols. 

In  the  red  the  band  narrows  gradually  with  addition  of  water,  its  limit 
in  the  solution  containing  no  water  being  at  X  7100,  and  reaching  the  limit  of 
the  sensibility  of  the  photographic  plates  used  in  the  solution  containing  18 
per  cent  of  water.  The  smaller  amount  of  absorption  here  as  compared  with 
the  methyl-alcohol  solutions  is  to  be  ascribed  to  the  shallower  layer  used. 

Copper  Nitrate  in  Water — Beer's  Law.    (See  Plate  44.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  4.04,  3.00,  2.00,  1.33,  0.92,  0.67,  and  0.50;  the  corresponding 
depths  of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the 
concentrations  were  0.67,  0.51,  0.34,  0.22,  0.15,  0.11,  and  0.084;  the  depths 
of  cell  were  the  same  as  in  A. 

The  most  concentrated  solutions  were  blue,  and  on  dilution  the  color 
changed  to  a  light  greenish-blue.  The  exposures  to  the  light  of  the  Nernst 
lamp  and  spark  were  of  1  and  3  minutes  duration,  respectively;  the  width 
of  the  slit  being  as  usual  0.01  cm. 

There  is  an  absorption  band  in  the  ultra-violet  distinct  from  the  NO, 
band  which  we  have  seen  before.  The  limit  of  transmission  for  the  most 
concentrated  solution  in  A  is  at  A  3600,  from  which  it  moves  gradually 
towards  the  region  of  shorter  wave-lengths  until  the  fifth  solution  is 
reached,  where  transmission  ceases  at  X  3390.    This  is  perhaps  the  limit  of 


56  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

the  NOg  band,  or  very  nearly  so  for  solutions  of  the  concentration  here 
used,  since  there  is  little  or  no  narrowing  of  the  absorption  in  passing  from 
the  fifth  to  the  seventh  strips  of  A.  In  B  the  transmission  is  sharply 
limited  by  the  NO3  band  throughout,  transmission  ceasing  at  X  3280. 

It  is  interesting  to  compare  the  absorption  of  this  salt  with  that  of 
copper  chloride.  The  latter  in  concentrated  solutions  absorbs  not  only 
the  ultra-violet,  but  also  all  of  the  violet  and  blue.  It  must  be  evident, 
then,  that  all  the  absorption  in  the  blue,  violet,  and  perhaps  also  the  ultra- 
violet, in  solutions  of  the  chloride  and  bromide,  is  to  be  ascribed  to  the 
molecule,  or  to  the  molecule  and  whatever  may  be  associated  with  it,  and 
not  in  any  way  to  the  ions.  The  copper  ions  very  likely  exert  no  absorp- 
tion on  light  of  such  short  wave-lengths  as  come  within  the  range  of  the 
present  investigation. 

In  the  red  the  most  concentrated  solution  of  A  absorbs  everything  of 
wave-length  longer  than  X  5980,  the  most  dilute  solution  of  A  transmitting 
some  light  as  far  out  as  X  6250.  For  B  the  limits  of  transmission  for  the 
most  concentrated  and  most  dilute  solutions  are,  respectively,  X  7200  and 
X  7250,  there  being  considerable  shading  from  about  X  6600  in  both  cases. 

Again  comparing  the  absorption  of  the  nitrate  and  the  chloride,  making 
due  allowances  for  differences  in  concentration,  we  find  that  the  red  band 
is  sensibly  the  same  for  the  two  salts — emphasizing  again  the  fact  that 
the  absorption  of  red  is  chiefly  a  function  of  the  concentration  of  copper 
atoms,  depending  only  to  a  slight  extent  on  their  immediate  surroundings. 

Copper  Nitrate  in  Water — Molectjles  Constant.     (See  Plate  45.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  4.04,  3.18,  2.36,  1.78,  1.38,  1.11,  and  0.92;  the  corresponding  depths 
of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.,  respectively.  The 
concentrations  for  B  were  1.00,  0.81,  0.60,  0.46,  0.347,  0.273,  and  0.223; 
the  depths  of  absorbing  layer  were  the  same  as  in  A.  The  exposures  to 
the  light  of  the  Nernst  lamp  and  spark  lasted  for  1  and  3  minutes,  respec- 
tively, the  slit  being  adjusted  to  a  width  of  0.01  cm. 

A  shows  that  the  absorption  in  the  ultra-violet  still  narrows  with  dilu- 
tion, the  limits  of  transmission  for  the  most  concentrated  and  most  dilute 
solutions  being,  respectively,  X  3600  and  X  3430.  In  B  the  transmission  is 
limited  by  the  NO3  band,  the  edge  of  which  falls  at  X  3280  throughout. 
Since  the  ultra-violet  absorption  narrows  even  in  this,  it  is  evident  that 
something  in  addition  to  the  simple  theory  of  dissociation  is  needed  to 
account  for  the  facts. 

In  the  red  we  find  that  the  band  first  narrows  until  the  third  strip  of 
A  is  reached,  then  widens  continuously  with  dilution,  the  edge  forming  a 
curved  line  concave  towards  the  violet.  The  limit  of  transmission  for  the 
first  solution  is  X  6000,  for  the  third  X  6050,  and  for  the  seventh  it  is  at 
X  5950.  In  B  the  absorption  increases  regularly  with  dilution,  the  limit 
in  the  first  solution  being  at  X  6800  and  at  X  6650  for  the  seventh.  There 
is  considerable  shading,  but  this  also  increases  somewhat  with  decrease 
in  concentration. 


SALTS    OF    COPPER.  57 

In  general,  the  absorption  spectrum  of  copper  salts  in  the  region  of 
the  spectrum  investigated  is  much  simpler  than  that  of  cobalt  salts,  inas- 
much as  it  presents  only  two  or  at  most  three  absorption  bands.  Of  these, 
only  the  one  at  X  4730  in  acetone  solutions  lies  wholly  in  the  spectral  region 
studied;  the  band  in  the  ultra-violet  is  what  might  be  termed  one-sided, 
no  region  of  transparency  on  its  more  refrangible  side  having  ever  been 
found.  The  band  in  the  red  is,  however,  strictly  a  band,  a  region  of  trans- 
parency existing  in  the  infra-red.  The  behavior  of  this  band  throughout 
strongly  suggests  the  green  band  of  cobalt  salts  in  solution,  while  the  ultra- 
violet absorption  is  somewhat  different  from  anything  we  have  found 
thus  far,  resembling  more  nearly  the  absorption  of  iron  salts,  to  be  dis- 
cussed in  the  next  chapter. 

Since  the  absorption  in  the  ultra-violet  decreases  rapidly  with  dilution, 
when  the  product  of  concentration  and  depth  of  layer  is  kept  constant, 
it  seems  reasonable  to  suppose  that  the  copper  ion  has  little  or  nothing  to 
do  with  it,  and  hence  that  it  must  be  ascribed  to  the  molecules;  but  as  the 
absorption  decreases  with  dilution,  even  when  molecules  are  kept  constant, 
without,  however,  entirely  disappearing  (as  was  the  case  with  some  cobalt 
bands),  we  must  conclude  that  the  absorbing  power  of  a  molecule  is  in- 
fluenced considerably  by  its  immediate  surroundings.  As  usual,  there  are 
at  least  two  possible  ways  of  explaining  the  increase  in  the  absorption 
with  concentration,  when  molecules  are  kept  constant.  One  is  to  assume 
the  formation  of  aggregates  of  molecules,  and  that  the  absorbing  power  of 
the  molecule  is  increased  thereby;  the  other  is  to  assume  the  existence  of 
solvates,  and  that  the  absorbing  power  of  a  molecule  decreases  with  increase 
in  the  complexity  of  the  solvate.  To  decide  between  these  two  possible 
explanations  we  need  only  take  into  account  the  change  in  the  absorption 
produced  by  a  rise  in  the  temperature  of  the  solution.  This  change  is  the 
same  qualitatively  as  that  produced  by  increasing  the  concentration. 
Molecular  aggregates  are  broken  down  by  rise  in  temperature,  and  hence, 
by  the  assumption  made  above  as  to  the  effect  of  aggregates  on  absorp- 
tion, this  should  decrease  the  absorption  instead  of  increasing  it.  We  must 
conclude,  therefore,  that  the  change  in  the  absorption  is  not  due  to  the 
formation  of  aggregates. 

Solvates  are  made  simpler  both  by  increasing  concentration  and  by 
rise  in  temperature;  and,  accordingly,  from  the  assumption  stated  above 
regarding  the  effect  of  complexity  of  solvates  on  absorption,  both  changes 
should  produce  similar  differences  in  the  absorption  spectrum,  which  is  in 
accordance  with  observed  facts.  We  conclude,  therefore,  that  the  ultra- 
violet absorption  of  solutions  of  copper  salts  is  due  to  the  "solvated"  mole- 
cules of  the  dissolved  salt,  and  that  the  absorbing  power  of  such  molecules 
is  decreased  as  the  complexity  of  the  solvate  increases. 

It  will  be  remembered  that  for  equal  concentrations  the  absorption  in 
the  region  of  shorter  wave-lengths  is  least  in  the  aqueous  solutions,  then 
increases  as  we  pass  from  methyl  alcohol  to  ethyl  alcohol.  In  general, 
also,  it  may  be  stated  that  the  change  in  the  absorption  with  dilution  is 
greatest  for  the  aqueous  solution,  and  then  decreases  as  we  pass  to  methyl 
and  ethyl  alcohols.    This  is  just  what  we  should  expect,  since  the  power  to 


58  ABSORPTION   SPECTRA   OP   SOLUTIONS. 

form  solvates  is  greater  with  water  than  with  either  of  the  alcohols,  and 
greater  for  methyl  than  for  ethyl  alcohol;  hence,  in  solutions  of  equal 
concentration  the  solvates  should  decrease  in  complexity  when  we  pass  in 
the  direction — water,  methyl  alcohol,  ethyl  alcohol.  Also,  on  dilution,  the 
change  in  the  complexity  of  the  solvate  should  be  greater  in  the  water 
solutions  than  in  the  methyl  or  ethyl  alcohol  solutions. 

The  absorption  band  in  the  red  narrows  somewhat  when  the  product 
of  concentration  and  depth  of  absorbing  layer  is  kept  constant,  but  widens 
when  molecules  are  kept  constant.  Its  intensity  does  not  change  nearly 
as  much  when  the  solvent  is  changed  as  was  the  case  with  the  violet  band, 
the  concentration  being  the  chief  factor  determining  it.  We  hence  con- 
clude that  this  band,  like  the  green  band  of  cobalt  salts,  is  due  to  the  metallic 
atom,  and  that  its  absorbing  power  is  affected  only  slightly  by  its  immediate 
surroundings.  The  peculiar  behavior  of  the  red  band  of  copper  chloride  in 
methyl  alcohol,  on  addition  of  water,  is  interesting,  especially  as  its  expla- 
nation is  not  at  all  difficult,  although  it  may  not  at  first  seem  so  simple. 
It  will  be  recalled  that  here  the  band  first  widened  when  water  was  added, 
and  then,  as  the  amount  of  water  was  increased,  it  narrowed  regularly. 
Now,  when  a  small  amount  of  water  is  added  it  is  reasonable  to  conclude 
that  the  hydrates  formed  would  be  comparatively  simple,  and  that  as  the 
amount  of  water  is  increased  their  complexity  also  increases. 

We  have  stated  that  the  effect  of  the  surroundings  on  the  absorption 
of  the  copper  atom  for  this  region  of  the  spectrum  is  sUght  compared  with 
what  it  is  in  the  ultra-violet;  but  that  it  is  quite  appreciable  is  shown  by 
the  narrowing  of  the  band  wherever  the  conditions  for  Beer's  law  hold. 
We  believe  that  this  narrowing  is  due  to  a  change  in  the  complexity  of  the 
solvates.  To  explain  the  case  in  question,  then,  we  need  only  say  that  the 
absorption  of  the  comparatively  simple  hydrates  formed  when  a  small 
amount  of  water  is  added  is  greater  than  that  of  the  methyl  alcoholates 
already  existing  in  the  original  solution;  and  that,  therefore,  the  first  effect 
observed  is  a  widening  of  the  band.  Its  subsequent  narrowing  as  more 
water  is  added  is,  of  course,  due  to  the  increase  in  the  complexity  of  the 
hydrates. 

In  the  similar  solutions  in  ethyl  alcohol  the  band  narrowed  from  the 
very  first,  which  is  just  what  we  should  expect,  since  the  solvates  here 
are  simpler  than  in  methyl  alcohol  and  hence  their  absorbing  power  would 
be  equal  to  or  greater  than  that  of  the  simple  hydrates  first  formed. 


CHAPTER  V. 

SALTS  OF  IRON. 

Ferric  Chloride  in  Water — Beer's  Law.    (See  Plates  46  and  47  B.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  Plate  46,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the 
numbered  scale,  were  1.30,  0.97,  0.65,  0.43,  0.30,  0.22,  and  0.16;  the  cor- 
responding depths  of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm. 
For  B,  Plate  46,  the  concentrations  were  0.22,  0.16,  0.11,  0.073,  0.051, 
0.037,  and  0.027;  and  for  B,  Plate  47,  0.037,  0.028,  0.018,  0.012,  0.0085, 
0.0062,  and  0.0048 ;  the  depths  of  cell  in  both  cases  were  the  same  as 
for  A,  Plate  46. 

The  most  concentrated  solutions  were  reddish-orange  as  viewed  in  their 
bottles,  from  which  the  color  changed  through  orange-yellow  to  nearly 
colorless  in  the  last  members  of  the  third  series.  The  exposure  to  the 
light  of  the  Nernst  lamp  lasted  for  1  minute,  the  slit  having  a  width  of 
0.01  cm.  No  exposure  to  the  light  from  the  spark  was  made,  as  it  was 
ascertained  by  a  preliminary  trial  that  even  the  most  dilute  solution  was 
opaque  in  the  entire  ultra-violet  beyond  X  3600.  A  trial  also  showed  com- 
plete transmission  in  the  red  as  far  as  k  7400,  and  hence  no  red-sensitive 
plates  were  used.  Hence,  all  the  spectra  of  iron  salts  shown  in  this  chapter 
seem  to  end  at  X  6000  or  near  there,  which  simply  shows  that  the  Seed 
film  used  was  not  sensitive  to  light  of  wave-length  longer  than  about  k  6000. 

The  absorption  decreases  fairly  regularly  with  dilution,  the  decrease 
being,  however,  less  at  high  dilutions.  The  limits  of  transmission  for  the 
most  concentrated  and  most  dilute  solutions  of  each  of  the  three  sets  are, 
respectively,  X  4850  to  X  4600,  X  4220  to  X  4100,  and  X  3900  to  X  3800,  show- 
ing that  whereas  the  narrowing  of  the  band  in  the  first  set  amounted  to 
250  A.U.,  in  the  third  set  it  was  only  100  A.U. 

An  attempt  was  made  to  carry  the  dilution  still  farther,  but  it  was 
found  that  the  very  dilute  solutions  changed  color  so  rapidly  with  time 
that  it  was  impossible  to  use  them  and  place  confidence  in  the  results. 

Ferric  Chloride  in  Water — Molecules  Constant.  (See  Plate  48.) 
The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  1.30,  1.00,  0.75,  0.53,  0.40,  0.30,  and  0.22,  the  corresponding 
depths  of  absorbing  layer  being  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B,  the 
concentrations  were  0.25,  0.20,  0.135,  0.10,  0.07,  0.05,  and  0.04;  the  depths 
of  cell  were  the  same  as  for  A. 

The  exposures  were  of  1  minute  duration,  and  were  made  to  the  light 
of  the  Nernst  lamp  only. 

It  will  be  seen  that  even  here  the  absorption  decreases  somewhat  with 
dilution,  the  narrowing  of  the  band  in  A  amounting  to  140  A.U.,  and  in  B 
to  about  100  units. 

59 


60  ABSORPTION    SPECTRA   OF    SOLUTIONS. 

Ferkic  Chloride  with  Calcium  Chloride.    (See  Plates  49  and  51  A.) 

The  concentration  of  ferric  chloride  in  the  solutions  used  in  making 
the  negative  for  A,  Plate  49,  was  constant  and  equal  to  0.182  normal. 
The  concentrations  of  calcium  chloride,  beginning  with  the  solution  whose 
spectrum  is  adjacent  to  the  scale,  were  3.97,  3.40,  2.85,  2.30,  1.75,  1.20, 
0.64,  and  0.00.  For  B  the  concentration  of  ferric  chloride  was  0.035,  and 
for  A,  Plate  51,  it  was  0.007;  the  concentrations  of  the  calcium  salt  were  the 
same  as  for  A,  Plate  49.    The  common  depth  of  absorbing  layer  was  1.5  cm. 

The  dilute  solutions  without  calcium  chloride  were  yellow,  or  very  faint 
yellow,  depending  upon  concentration.  With  increasing  amount  of  the 
calcium  salt  the  color  deepened  very  markedly,  becoming  orange  to  reddish- 
orange,  according  to  the  concentration  of  the  colored  salt. 

The  spectrograms  show  the  marked  increase  in  width  of  the  absorp- 
tion band  with  addition  of  the  dehydrating  agent.  In  A  the  solution 
containing  no  calcium  chloride  transmits  as  far  as  X  4600,  while  the  one 
containing  the  greatest  amount  of  the  calcium  salt  ceases  to  transmit  at 
}.  5250.  For  B  the  corresponding  wave-lengths  are  k  4150  and  A  4950,  and 
for  A,  Plate  51,  they  are  X  3860  and  X  4620,  respectively. 

In  each  case  the  line  formed  by  the  limits  of  transmission  is  curved, 
with  its  concave  side  towards  the  region  of  short  wave-lengths,  showing 
that  the  absorption  decreases  most  rapidly  at  first  with  addition  of  the 
calcium  salt.  The  increments  in  the  concentration  of  the  dehydrating 
agent  from  solution  to  solution  were  sensibly  the  same,  namely  0.55  normal. 

Ferric  Chloride  with  ALUMmiUM  Chloride.     (See  Plates  50  and  61  B.) 

The  concentration  of  the  iron  salt  in  the  solutions  used  in  making  the 
negative  for  A,  Plate  50,  was  constant  and  equal  to  0.182  normal.  The 
concentrations  of  aluminium  chloride,  beginning  with  the  solution  whose 
spectrum  is  adjacent  to  the  numbered  scale,  were  2.61,  2.25,  1.88,  1.52, 
1.16,  0.79,  0.43,  and  0.00;  the  successive  increments  in  concentration 
were  all  0.366,  except  the  last,  which  is  0.43.  For  B  the  concentration  of 
ferric  chloride  was  0.035,  and  for  B,  Plate  51,  it  was  0.007,  the  concentra- 
tions of  the  aluminium  chloride  being  the  same  as  for  A,  Plate  50. 

The  common  depth  of  absorbing  layer  was  1.5  cm.,  and  the  exposure 
which  was  made  to  the  light  of  the  Nernst  lamp  lasted  only  1  minute, 
the  slit  having  the  usual  width  of  0.01  cm. 

The  spectrograms  are  very  similar  to  those  made  with  the  solutions 
containing  calcium  chloride  as  dehydrating  agent,  the  only  difference 
being  the  somewhat  greater  widening  of  the  absorption  band  in  the  pres- 
ent case.  Since  the  concentrations  of  the  iron  salt  were  the  same,  set  for 
set,  with  calcium  chloride,  as  in  the  present  case,  the  spectrograms  are 
directly  comparable. 

The  limits  of  transmission  for  the  solutions  containing  no  dehydrating 
agent  are  of  course  the  same  in  the  two  cases,  as  we  need  only  compare 
the  limits  for  the  solutions  containing  the  greatest  amount  of  the  calcium 
or  aluminium  salt.  We  find  that  in  the  series  containing  the  greatest 
amount  of  the  iron  salt,  the  limit  of  transmission  is  50  A.U.  nearer  the  red 
end  of  the  spectrum  for  the  solution  containing  the  aluminium  salt  than  for 


SALTS    OF    IRON.  61 

the  one  containing  the  calcium  salt.  For  the  solution  containing  ferric  chlo- 
ride, at  concentration  0.035,  the  difference  is  again  50  A.U.,  while  for  the 
one  having  the  concentration  0.007  the  difference  amounts  to  80  units. 

Ferric  Chloride  in  Methyl  Alcohol — Beer's  Law.    (See  Plates  52  and  54  A.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  Plate  52,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the 
numbered  scale,  were  1.23,  0.923,  0.615,  0.410,  0.284,  0.205,  and  0.135; 
the  corresponding  depths  of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and 
24  mm.  For  B,  Plate  52,  the  concentrations  were  0.2,  0.15,  0.1,  0.066, 
0.046,  0.034,  and  0.025,  and  for  A,  Plate  54,  they  were  0.034,  0.026,  0.017, 
0.011,  0.0078,  0.0057,  and  0.0042;  the  depths  of  cell  were  the  same  as  in 
A,  Plate  52. 

The  most  concentrated  solutions  were  deep  orange-red,  from  which  on 
dilution  the  color  changed  to  a  clear  yellow. 

The  exposure  which  was  made  to  the  Nernst  lamp  lasted  only  1  minute, 
the  slit  having  the  usual  width  of  0.01  cm.  No  exposure  was  made  for 
the  red  end  of  the  spectrum,  as  examination  by  the  direct-vision  spectro- 
scope showed  no  absorption  in  this  region. 

The  three  spectrograms  show  that  Beer's  law  holds  very  accurately 
over  the  range  of  concentrations  studied,  the  edge  of  the  absorption  band 
remaining  unchanged  in  position  in  any  one  series.  In  A,  Plate  52,  the 
limit  of  transmission  is  at  A  5300,  in  B  at  A  4950,  and  in  A,  Plate  54,  it  falls 
at  X  4600. 

In  A,  Plate  46,  the  limit  of  transmission  was  not  far  from  X  4700.  The 
concentrations  and  depths  of  cell  there  were  about  the  same  as  in  A,  Plate 
52,  while  the  solvent  there  was  water  and  here  methyl  alcohol.  This  indi- 
cates considerably  greater  absorbing  power  for  the  salt  when  dissolved 
in  methyl  alcohol,  if,  as  is  usual,  the  actual  shift  of  the  center  of  the  absorp- 
tion band  is  not  very  great.  In  the  present  case,  since  Beer's  law  holds, 
we  may  assume  that  all  the  moving  parts  containing  an  iron  atom  are 
equally  active  in  absorbing  light;  while  in  the  aqueous  solution,  since  Beer's 
law  does  not  hold,  some  of  them  must  either  not  absorb  at  all,  or  else  much 
more  feebly  than  others. 

Ferric  Chloride  in  Ethyl  Alcohol — Beer's  Law.    (See  Plates  53  and  54  B.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  Plate  53,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the 
numbered  scale,  were  1.23,  0.923,  0.62,  0.41,  0.28,  0.21,  and  0.15;  the 
corresponding  depths  of  cell  being  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B, 
Plate  53,  the  concentrations  were  0.20,  0.15,  0.10,  0.066,  0.046,  0.034,  and 
0.025;  and  for  B,  Plate  54,  0.034,  0.026,  0.017,  0.011,  0.0078,  0.0057,  and 
0.0042 ;  the  depths  of  absorbing  layer  were  the  same  as  in  A,  Plate  53. 

The  color  of  these  solutions  was  identical  with  that  of  the  solutions 
in  methyl  alcohol.  The  exposure,  which  was  made  to  the  light  of  the  Nernst 
lamp,  lasted  only  for  1  minute ;  the  slit  had  a  width  of  0.01  cm. 

It  will  be  seen  that  here  also  Beer's  law  holds  fairly  well,  the  deviation 
from  it  in  the  most  concentrated  series  causing  the  band  to  narrow  by 


62  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

something  like  only  40  A.U.  In  the  intermediate  series  the  narrowing  of 
the  band  with  dilution  is  still  less,  only  about  25  or  30  A.U.,  and  so  may 
very  probably  be  due  to  a  gradual  shift  in  the  position  of  the  film  during 
the  exposure.  In  the  most  dilute  series  no  change  in  the  width  of  the 
region  of  absorption  can  be  noted.  The  slight  deviation  observed  in  the 
first  series  may  perhaps  be  due  to  mutual  influence  of  the  absorbers,  as 
pointed  out  in  the  introductory  chapter. 

Comparing  these  spectrograms  with  those  of  solutions  in  methyl  alco- 
hol, we  find  that  the  limit  of  transmission  here  is  always  a  little  nearer 
the  region  of  short  wave-lengths,  which  is  a  little  different  from  what  we 
have  usually  found.  The  rule  has  been  that  the  absorbing  power  of  any  sub- 
stance in  ethyl  alcohol  is  somewhat  greater  than  in  methyl  alcohol,  while 
here  the  opposite  is  true.  Ferric  chloride  is,  however,  a  rather  unstable  sub- 
stance in  solution,  and  it  is  possible  that  the  anomalies  which  we  have  noted 
are  due  to  some  chemical  change  which  has  not  been  taken  into  account. 

Fkrbic  Chloeide  in  Acetone — Beer's  Law.    (See  Plate  55.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  0.086,  0.064,  0.043,  0.029,  0.020,  0.014,  and  0.011;  the  corresponding 
depths  of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the 
concentrations  were  0.014,  0.010,  0.007,  0.0047,  0.0032,  0.0023,  and  0.0017, 
the  depths  of  cell  being  the  same  as  for  A. 

The  most  concentrated  solutions  were  red,  from  which  on  dilution  the 
color  changed  to  yellow.  The  exposure,  which  was  made  only  to  the  light 
from  the  Nernst  lamp,  lasted  1  minute,  the  slit  having  the  usual  width 
of  0.01  cm. 

It  was  observed  that  a  solution  of  ferric  chloride  in  acetone,  on  being 
allowed  to  stand,  changes  color  slowly  with  time,  the  color  becoming 
deeper.  A  solution  which  when  freshly  made  up  was  yellow,  was  found 
to  be  a  clear  orange  after  two  days.  In  order  to  obtain  the  spectra  photo- 
graph before  any  appreciable  change  took  place,  it  was  necessary  to  make 
the  exposure  just  as  soon  as  the  solution  was  made  up,  and  this  was  done, 
the  time  elapsing  between  making  up  the  series  and  completing  the  expos- 
ures for  the  spectrogram  being  not  more  than  30  minutes. 

The  negative  for  A  shows  a  decrease  of  absorption  with  dilution,  while 
B  shows  no  change  in  the  width  of  the  band.  This  is  the  same  as  what 
we  just  found  in  the  case  of  the  solutions  in  ethyl  alcohol,  only  the  narrow- 
ing of  the  band  in  the  concentrated  solutions  is  much  greater  with  acetone. 


CHAPTER  VI. 

SALTS   OF  CHROMIUM. 

A  comparatively  large  number  of  investigators  have  worked  on  salts 
of  chromium  from  the  standpoint  of  absorption  of  light.  We  need  only- 
mention  the  work  of  Talbot/  Brewster,^'  Croft,^  Miiller,*  Gladstone,^  Melde,* 
the  early  work  of  Hartley,'  Vierordt,*  Vogel,'  E.  Wiedermann/"  Soret," 
Settegast,"  Moissan/^  Pulfrich,"  Zimmermann/^  Becquerel,*"  Liveing 
and  Dewar,"  Schunck,"  Recoura/*  and  Sabatier.^" 

Knoblauch/*  in  his  interesting  and  important  investigation  on  the  ab- 
sorption spectra  of  very  dilute  solutions,  studied  a  number  of  chromium 
compounds.  These  were  the  chloride,  nitrate,  sulphate,  acetate,  oxalate, 
potassium  chrom-oxalate,  and  chrom-alum.  Knoblauch  directed  a  part 
of  his  work  to  testing  the  consequences  of  the  then  recently  proposed 
theory  of  electrolytic  dissociation.  According  to  this  theory,  the  absorp- 
tion spectrum  of  a  concentrated  solution  must  be  different  from  that  of 
a  very  dilute  solution;  and  at  dilutions  of  complete  dissociation,  all  of 
the  salts  of  an  acid  with  a  colored  anion,  having  colorless  cations,  or  all 
of  the  salts  of  a  metal  having  colorless  anions,  must  have  the  same  absorp- 
tion spectrum.  Knoblauch  found  that  neither  of  these  conclusions  from 
the  theory  was  verified  experimentally. 

Ostwald  "  showed  a  little  later  that  the  second  consequence  of  the 
theory  is  fully  verified  by  experimental  facts. 

Knoblauch  also  tested  Beer's  law,  and  found  that  it  held  for  many 
salts  within  wide  limits  of  concentration.  He  concluded  that  the  apparent 
deviations  from  the  law  are  to  be  explained  as  due  to  chemical  or  physical 
changes  in  the  solutions. 

^  Phil.  Mag.  (3),  4,  112  (1834). 

2  Phil.  Trans.,  1835,  1,  91,  and  Phil.  Mag.  (4),  24,  441  (1862). 

» Ibid.  (3),  21,  197  (1842). 

♦  Pogg.  Ann.,  72,  76  (1847),  and  79.  344  (1850). 

» Phil.  Mag.  (4),  14,  418  (1857),  and  Journ.  Chem.  Soc.,  10,  79  (1858). 

•  Pogg.  Ann.,  124,  91  (1865). 

'  Proc.  Roy.  Soc,  21,  499  (1873). 

« Ber.  d.  deutsch.  chem.  Gesell.,  5,  34  (1872). 

» Ibid.,  11,  913,  1363  (1878). 
>»  Wied.  Ann.,  5,  500  (1878). 

"  Arch.  Sci.  Phys.  et  Nat.  (2),  61,  322  (1878);  (2),  63,  89  (1878). 
"  Wied.  Ann.,  7,  242  (1879). 
"  Compt.  rend.,  93,  1079  (1881). 
"  Ztschr.  f .  Kryst.,  6,  142  (1882). 
"  Lieb.  Ann.,  213,  285  (1882). 
"  Ann.  Chim.  Phys.  (5),  30,  5  (1883). 
"Proc.  Roy.  Soc,  35,  71  (1883). 
"Chem.  News,  51,  152  (1885). 
"Compt.  rend.,  102,  515  (1886),  112,  1439  (1891). 
»Ibid.,  103,49  (1886). 
»  Wied.  Ann.,  43,  738  (1891). 
22  Ztschr.  phys.  Chem.,  9,  579  (1892). 

63 


64 


ABSORPTION  SPECTRA  OF  SOLUTIONS. 


Hartley,*  in  his  elaborate  investigations  on  absorption  spectra,  studied 
a  number  of  salts  of  chromium.  The  salt  Cr(N03)3.9H20  is  violet  in  color. 
When  heated  to  100°  it  changes  to  green.  Hartley  concludes  that  the  violet 
are  the  normal  salts  of  chromium,  while  the  green  are  chromyl  salts,  thus: 


Violet  salts. 

Green  salts. 

Chloride 

CrCla.SH^O 

Cr,(N0^,.9H,0 

Cr,0Cl,.2H,0 

CrjO(SO,),.XH20 

Cr,0(N03)4.H30 

Sulphate 

Nitrate 

An  interesting  paper  by  Vernon '  on  "  The  Dissociation  of  Electrolytes 
in  Solution  as  Shown  by  Colorimetric  Determinations,"  was  published  in 
sections  in  the  Chemical  News.  Chromium  plays  in  this  work  what  might 
be  termed  the  exceptional  role,  as  is  seen  from  the  following  conclusions 
drawn  by  the  author  :  ^ 

Almost  all  the  solutions  of  the  thirty-five  colored  salts  examined  show  considerable 
decrease  in  color  effect  on  dilution,  due  in  all  probability  to  dissociation  taking  place.  The 
only  exceptions  are  certain  chromiimi  derivatives,  the  color  of  whose  solutions,  on  gradual 
dilution,  either  remains  constant  or  increases  slightly.  All  solutions  of  salts,  with  the  excep- 
tion of  those  of  certain  chromivun  derivatives  and  perhaps  of  potassium  permanganate 
increase  considerably  in  color  effect  on  being  heated. 

Lapraik  *  has  described  a  large  number  of  empirical  relations  between 
the  absorption  spectra  of  compounds  of  chromium.  Certain  classes  of 
chromium  compounds  were  found  to  have  the  same  absorption  spectra  in 
solution  and  in  the  solid  state.  The  band  A  710  to  A  692  is  present  in  all 
of  the  chromium  compounds  investigated,  with  the  exception  of  potassium 
chromium  cyanide.  In  certain  compounds,  however,  the  band  is  displaced 
somewhat  from  the  above  position. 

The  broad  absorption  in  the  green  in  the  region  of  X  550  to  X  650  is  present 
in  all  chromium  compounds,  sometimes  displaced  somewhat  towards  the  red, 
in  other  compounds  towards  the  blue,  as  referred  to  the  above  wave-lengths. 

Etard  *  studied  the  sulphate  of  chromium,  chrome  alum,  and  violet 
chromium  nitrate.  He  found  a  band  in  the  red  which  was  characteristic 
of  chromium  salts,  and  which  extended  from  X  670  to  X  678.  He  concluded 
that  the  absorption  spectra  of  chromium  salts  are  due  to  the  molecules. 

The  absorption  bands  produced  by  an  element  are  displaced  in  position,  or 
may  cease  to  exist  entirely,  depending  upon  the  nature  of  the  whole  molecule 
which  is  in  solution,  i.e.,  upon  the  particular  compound  of  the  element  studied. 

Chromium  Chlobide  in  Water — Beer's  Law.     (See  Plate  56  A.) 

The  concentrations  of  the  solutions,  beginning  with  the  one  whose 
spectrum  is  adjacent  to  the  numbered  scale,  were  0.53,  0.40,  0.26,  0.17, 
0.12,  0.09,  and  0.07;  the  corresponding  depths  of  absorbing  layer  were 
3,  4,  6,  9,  13,  18,  and  24  mm. 

1  Chem.  News,  65,  15  (1S92).    Dublin  Trans.  (2).  7,  253  (1900). 

»Chem.  News,  66,  104,  114,  141,  and  152  (1892). 

» Ibid.,  66,  154  (1892). 

*  Joum.  prakt.  Chem.,  47,  305  (1893). 

•Compt.  rend.,  120,  1057  (1895). 


SALTS    OF    CHROMIUM.  65 

The  exposures  to  the  hght  of  the  Nernst  lamp  and  spark  lasted,  respec- 
tively, IJ  and  3  minutes,  the  slit  having  a  width  of  0.01  cm. 

In  deep  layers  the  more  concentrated  solutions  were  red,  while  in  shal- 
low layers  they  were  green.  The  dilute  solutions  appeared  green  as  seen 
in  their  bottles. 

The  spectrogram  shows  three  regions  of  absorption.  One  is  located  in 
the  extreme  ultra-violet,  cutting  off  the  part  of  the  spectrum  lying  to 
the  more  refrangible  side  of  X  2840.  Another  absorption  band  is  in  the 
violet,  its  more  refrangible  limit  being  at  X  3900  and  moderately  well 
defined,  while  the  red  edge,  which  is  more  hazy,  is  near  X  4450.  A  third 
band  is  located  in  the  yellow  and  orange.  Both  edges  of  this  band  are 
very  diffuse,  the  red  edge  slightly  more  than  the  blue.  The  spectrograms 
indicate  the  opposite  to  be  the  case,  but  this  is  due  to  the  fact  that  the  blue 
edge  of  the  band  falls  near  the  region  of  minimum  sensibility  of  the  Seed 
film,  and  hence  the  shading  there  is  very  much  accentuated.  The  limits 
of  transmission  are  at  X  5650  and  about  X  6200,  although  there  is  consid- 
erable absorption  as  far  "down"  as  X  5200  and  as  far  up  as  X  6750.  From 
X  6800  to  the  end  of  the  visible  red  the  solutions  are  remarkably  transparent, 
which  was  made  very  evident  by  the  fact  that  a  concentrated  solution, 
about  20  cm.  in  depth,  still  transmitted  red  light  freely,  although  not  a 
trace  of  green  could  be  seen  through  a  layer  5  cm.  deep. 

The  width  of  all  these  bands  is  absolutely  unchanged  by  change  in 
dilution,  within  the  limits  of  concentration  here  used,  showing  that  the 
absorption  is  strictly  proportional  to  the  number  of  chromium  atoms  in 
the  path  of  the  beam  of  light,  and  independent  of  whether  they  exist  as 
ions  or  combined  with  other  atoms  in  a  molecule. 

CHROMnjM  Chloride  in  Water — Molecules  Constant.  (See  Plate  66  B.) 
The  concentrations  of  the  solutions,  beginning  with  the  one  whose  spec- 
trum is  adjacent  to  the  numbered  scale,  were  0.45,  0.35, 0.245,  0.173,  0.125, 
0.093,  and  0.074;  the  corresponding  depths  of  absorbing  layer  were  3,  4, 6,  9, 
13, 18,  and  24  mm.  The  exposures  to  the  light  of  the  Nernst  lamp  and  spark 
lasted,  respectively,  IJ  and  3  minutes;  the  width  of  the  slit  was  0.01  cm. 
The  spectrogram  shows  the  same  regions  of  absorption  as  A,  but  in 
this  case  they  all  widen  with  decrease  in  concentration,  as  was  to  be 
expected.  The  bands  all  widen  to  nearly  the  same  extent.  The  shading 
on  the  red  side  of  the  least  refrangible  band  does  not  extend  any  farther 
into  the  red  with  decrease  in  concentration,  indicating  that  this  band  has 
a  fairly  well-defined  limit  near  X  6800. 

A  rather  narrow  band  of  absorption  may  be  seen  at  X  6690.  It  is  faint 
and  scarcely  noticeable  in  A.  Its  width  is  not  easy  to  determine,  but 
appears  to  be  only  about  30  A.U. 

Chromium  Chloride  with  Calcium  Chloride  and  Aluminium  Chloride. 

(See  Plate  57.) 

The  concentration  of  the  chromium  chloride  was  constant  throughout 

and  equal  to  0.074.     The  concentrations  of  calcium  chloride,  beginning 

with  the  solution  whose  spectrum  is  adjacent  to  the  numbered  scale  of  A, 

were  3.96,  3.45,  2.85,  2.30,  1.75,  1.20,  0.64,  and  0.0.    The  concentrations 

6 


66  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

of  aluminium  chloride  in  the  solutions  used  in  making  B  were,  taken  in 
the  same  order,  2.58,  2.25,  1.86,  1.50,  1.14,  0.78,  0.42,  and  0.0. 

The  common  depth  of  absorbing  layer  was  1.5  cm.,  and  the  times  of 
exposure  to  the  light  of  the  Nernst  lamp  and  spark  were,  respectively, 
1|  and  3  minutes;  the  slit  had  the  usual  width  of  0.01  cm. 

The  effect  of  adding  these  dehydrating  agents  is  to  widen  all  the  absorp- 
tion bands,  the  widening  seeming  to  increase  a  little  more  rapidly  the 
more  concentrated  the  solution  of  the  dehydrating  agent.  This  gives  the 
edges  of  the  bands  a  slightly  rounded  appearance. 

It  is  also  noticed  that  the  effect  of  the  aluminium  salt  is  a  little  greater 
than  that  of  the  calcium  salt,  although  the  concentrations  were  so  chosen 
as  to  make  the  number  of  chlorine  atoms  added  as  nearly  the  same  as 
possible.  The  widening  can  not  be  due  to  a  driving  back  of  the  dissocia- 
tion of  the  chromium  salt,  since  we  have  just  seen  that  the  absorption  of 
the  chromium  chloride  does  not  in  any  way  vary  with  its  dissociation.  The 
most  probable  explanation  here  as  elsewhere  is  that  some  simple  hydrates 
are  formed,  which  normally  do  not  exist  except  in  very  concentrated  solu- 
tions or  at  high  temperatures,  and  that  the  absorbing  power  of  a  chromium 
atom  thus  hydrated  is  greater  than  when  the  hydrate  is  more  complex. 

The  band  in  the  red  at  A  6690  shows  faintly  on  the  negatives  for  both  these 
spectrograms,  although  it  can  not  be  seen  in  the  reproduction.  It  does  not 
seem  to  be  affected  to  any  appreciable  extent  by  addition  of  the  foreign  salts. 

Chbomium  Nitrate  in  Water — ^Beer's  Law.    (See  Plate  58  A.) 

The  concentrations  of  the  solutions,  beginning  with  the  one  whose 
spectrum  is  adjacent  to  the  numbered  scale,  were  0.754,  0.564,  0.377,  0.251, 
0.174,  0.126,  and  0.094;  the  corresponding  depths  of  absorbing  layer  were 
3,  4,  6,  9,  13,  18,  and  24  mm.  The  exposures  to  the  light  of  the  Nernst 
lamp  and  spark  lasted,  respectively,  \\  and  3  minutes,  the  slit  having  the 
usual  width  of  0.01  cm. 

The  solutions  of  the  nitrate  are  similar  in  color  to  those  of  the  chloride 
already  described,  excepting  that  the  latter  is  relatively  more  transparent 
in  the  green  and  less  so  in  the  red.  The  result  is  that  in  layers  of  any 
depth  the  nitrate  solutions  are  more  apt  to  show  red,  especially  in  gas- 
light. Dilute  solutions  or  very  thin  layers  of  concentrated  solutions  are 
greenish  in  color. 

The  spectrogram  shows  the  same  absorption  bands  as  we  have  already 
found  and  discussed  for  the  chloride.  Owing  to  the  fact  that  the  concen- 
tration of  the  solutions  of  the  nitrate  was  somewhat  greater,  the  bands 
are  wider  and  their  edges  are  much  sharper. 

In  the  ultra-violet  the  transmission  is  sharply  limited  by  the  NO3  band, 
and  hence  we  find  absorption  complete  from  X  3270  to  the  end  of  the 
spectrum. 

In  the  most  concentrated  solution,  the  violet  band  begins  at  X  3710 
and  ends  at  X  4450,  these  figures  being  the  limits  of  (photographic)  trans- 
mission. Owing  to  the  slight  shading  the  absorption  extends  somewhat 
farther  to  both  sides.  In  the  most  dilute  solutions  the  limits  are  X  3710 
and  X  4420,  showing  a  slight  narrowing  of  the  band  from  the  red  side. 


SALTS    OF    CHROMIUM.  67 

The  limits  for  the  yellow  band  in  the  most  concentrated  solution  are 
A  5170  and  A  6220,  while  for  the  most  dilute  they  are  X  5250  and  X  6150, 
showing  considerable  narrowing  with  decrease  in  concentration. 

It  will  be  noticed  that  both  the  bands  are  somewhat  nearer  the  region 
of  shorter  wave-lengths  in  the  nitrate  than  in  the  chloride,  the  centers  of 
the  two  bands  for  the  latter  being  ^^  4175  and  X  5925,  while  for  the  former 
the  corresponding  figures  are  X  4065  and  X  5700.  The  yellow  band  in  the 
nitrate  is  also  much  hazier  on  its  violet  side  than  was  the  case  with  the 
chloride.  This  is  not  brought  out  by  the  reproduction,  or  even  by  the 
negative  made  with  the  Seed  film,  on  account  of  the  fact  that  the  edge  of 
the  band  falls  so  near  the  middle  of  the  minimum  in  the  sensibility  curve 
of  the  Seed  emulsion.  A  negative  made  on  a  Wratten  panchromatic  plate, 
giving  both  edges  of  the  band,  however,  shows  very  clearly  the  difference 
in  the  shading,  this  being  nearly  twice  as  great  on  the  violet  side  as  on 
the  red. 

The  negative  shows  the  band  at  X  6690  rather  better  than  did  those 
made  with  the  chloride  solutions.  The  band  is  rather  faint,  but  may  be 
seen  well  enough  to  enable  one  to  determine  its  position  and  general  char- 
acter. It  remains  of  constant  width  and  intensity  throughout.  Its  maxi- 
mum of  intensity  falls  very  close  to  X  6690,  from  which  position  a  gradual 
shading  extends  to  a  distance  of  about  20  A.U.  on  both  sides.  Except  for 
this  band,  the  solutions  are  almost  perfectly  transparent  from  X  6400  to 
the  end  of  the  visible  red. 

Chromium  Nitrate  in  Water — ^Molecules  Constant.    (See  Plate  58  B.) 

The  concentrations  of  the  solutions,  beginning  with  the  one  whose 
spectrum  is  adjacent  to  the  numbered  scale,  were  0.70,  0.55,  0.39,  0.28,  0.20, 
0.15,  and  0.12;  the  corresponding  depths  of  cell  were  3, 4, 6,  9, 13, 18,  and  24 
mm.    The  exposures  and  slit  width  were  the  same  as  used  with  Plate  58  A. 

The  spectrogram  shows  the  same  bands  as  Plate  58  A,  only  here  they 
all  widen  somewhat  with  dilution,  as  was  to  be  expected  from  their  be- 
havior when  the  conditions  for  Beer's  law  were  fulfilled. 


CHAPTER  VII. 

SALTS  OF  NEODYMIUM,  PRASEODYMIUM, 
AND  ERBIUM. 

Some  of  the  more  important  investigations  on  the  salts  of  the  above- 
named  elements  are  the  following: 

Bahr  and  Bunsen  ^  in  their  early  work  on  absorption  spectra  included 
didymium  and  erbium. 

Becquerel,^  in  his  study  of  spectra  in  the  infra-red  region,  worked  with 
didymium.  In  his  subsequent  work '  on  the  variation  of  absorption  spectra 
in  crystals,  the  sulphate  and  nitrate  of  didymium  were  included. 

Becquerel  *  compared  the  absorption  spectra  of  crystals  of  didymium 
salts  with  the  spectra  of  the  aqueous  solution  of  the  same  salts.  He  showed 
that  from  the  displacement  of  the  bands  he  could  recognize  distinct  sub- 
stances or  definite  compounds.  He  showed  that  in  the  crystals  we  may 
have,  simultaneously,  mixtures  of  different  compounds  and  especially 
basic  salts. 

Demar^ay  *  studied  the  spectrum  of  didymium,  and  concluded  that  in 
addition  to  praseodymium  and  neodymium  there  was  probably  present  a 
third  element.  In  a  subsequent  investigation "  he  shows  that  neodymium 
from  entirely  different  minerals  and  sources  always  has  the  same  spec- 
trum, and  concludes  that  it  is  a  chemical  element. 

Kriiss  and  Wilson '  carried  out  an  elaborate  investigation  on  the  absorp- 
tion spectra  of  the  rare  earths.  They  concluded  that  we  must  assume  the 
existence  of  more  than  twenty  elements  in  the  various  rare  earth  minerals. 

Bettendorff*  carried  out  three  investigations  on  the  spectra  of  the 
cerium  and  yttrium  group,  and  Schottlander  *  made  use  of  his  spectro- 
scopic studies  and  spectrophotometric  work  to  characterize  the  various 
rare  earths. 

Boudouard  "*  effected  the  separation  of  neodymium  and  praseodymium  by 
means  of  potassium  sulphate  instead  of  ammonium  nitrate.  The  absorp- 
tion spectra  indicated  a  nearly  complete  separation  from  praseodymium. 

Scheele "  did  some  very  careful  work  on  praseodymium  in  connection 
with  his  determination  of  the  atomic  weight  of  that  element,  and  later  " 
in  connection  with  the  separation  of  praseodymium  and  neodymium. 

'  Lieb.  Ann.,  137,  1  (1886). 

» Ann.  Chim.  Phys.  (5),  30,  5  (1883). 

•Ibid.  (6),  14,  170  (1888). 

*  Ibid.  (6),  14,  257  (1888). 

» Compt.  rend.,  102,  1551  (1886),  105,  276  (1887). 

'Ibid.,  126,  1040  (1898). 

'  Ber.  d.  deutsch.  chem.  Gesell.,  20,  2134  (1887). 

8  Lieb.  Ann.,  256,  159^(1890);  263,  164  (1891);  270,  376  (1892). 

•Ber.  d.  deutsch.  chem.  Gesell.,  25,  569  (1892). 
10  Compt.  rend.,  126,  900  (1898). 
"Ztschr.  anorg.  Chem.,  17,  310  (1898). 
'2  Ber.  d.  deutsch.  chem.  Gesell.,  32,  409  (1899). 

68 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,    AND    ERBIUM.  69 

The  elaborate  investigations  of  Muthmann  ^  and  his  coworkers,  Stiitzel, 
Bohm,  Baur,  Hofer,  and  Weiss,  call  for  special  comment.  They  raise  the 
question  as  to  the  elementary  nature  of  praseodymium  and  neodymium, 
and  point  out  certain  lines  of  evidence  based  on  spectrum  analysis  which 
make  it  not  impossible  that  these  substances  are  mixtures.  They  show 
that  by  spectrum  analysis  it  is  possible  to  determine  approximately  the 
amounts  of  neodymium  and  praseodymium  in  a  mixture  of  the  two,  a 
fact  which  had  earlier  been  utilized  by  Jones  ^  in  his  work  on  the  atomic 
weights  of  these  two  elements. 

By  the  electrolysis  of  fused  neodymium  compounds  Muthmann  was 
able  to  prepare  the  pure  metal.  He  then  studied  the  physical  properties 
not  only  of  neodymium,  but  also  of  cerium,  lanthanum,  and  praseodym- 
ium, which  were  prepared  in  the  same  manner. 

An  important  and  interesting  investigation  was  carried  out  by  Liveing ' 
on  the  effects  of  dilution,  temperature,  nature  of  the  solvent,  etc.,  on  the 
absorption  spectra  of  solutions  of  didymium  and  erbium  salts.  His  eye 
observations  were  made  with  an  ordinary  spectroscope,  and  the  photo- 
graphs also  with  a  prism  spectroscope.  He  obtained  some  very  good  plates, 
indeed,  considering  the  kind  of  apparatus  employed.  He  studied  the  effect 
on  the  absorption  of  increasing  the  dilution  of  the  solution,  and  established 
the  four  following  facts: 

The  spectra  of  the  different  salts  of  the  same  metal  in  dilute  solution 
are  identical.  The  spectrum  is  constant  for  the  chloride  and  sulphate  in 
different  dilutions,  as  long  as  the  thickness  of  the  absorbing  solutions  is 
proportional  to  the  dilution.  The  spectrum  of  the  nitrate  is  modified  by 
some  cause  with  increasing  concentration. 

The  absorption  of  the  short  wave-lengths,  which  differ  for  different 
salts,  diminishes  with  increased  dilution. 

The  effect  of  rise  in  temperature  from  about  20°  to  97°  renders  the  bands 
more  diffuse,  but  does  not  increase  their  intensity. 

The  addition  of  acid  made  the  absorption  in  general  more  diffuse,  but 
did  not  weaken  the  absorption. 

From  this  fact,  together  with  the  fact  that  rise  in  temperature  does  not 
increase  the  intensity  of  absorption,  Liveing  concluded  that  absorption 
can  not  be  accounted  for  on  the  basis  of  electrolytic  dissociation. 

The  work  of  Liveing  on  absorption  in  non-aqueous  solvents  is  of  special 
interest  in  the  present  connection. 

He  says  that  didymium  chloride  dried  at  a  temperature  above  100°  is 
quite  insoluble  in  alcohol.  This  was  doubtless  due  to  the  formation  of  the 
basic  chloride.  This  salt  can  be  heated  to  140°  to  150°  in  a  current  of  dry 
hydrochloric  acid  gas,  and  the  anhydrous  salt  is  still  perfectly  soluble  in  alco- 
hol.   The  salt  with  which  he  worked  doubtless  contained  more  or  less  water. 

Liveing  says  that  the  absorption  spectrum  of  the  alcoholic  solution 
shows  the  same  bands  as  an  aqueous  solution,  but  they  are  somewhat 

>  Ber.  d.  deutsch.  chem.  Gesell.,  32,  2653  (1899);    33,  42,  1748,  2028  (1900);    Lieb. 

Ann.,  320,  231  (1902). 
» Amer.  Chem.  Joum.,  20,  345  (1898). 
^Camb.  Phil.  Soc,  18,  298  (1900). 


70  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

modified.  The  positions  of  maximum  absorption  are  all  moved  towards 
the  red.  The  shift  of  the  different  bands  is  not  equal.  The  bands  in  the 
yellow  and  green  in  the  alcoholic  solution  are  so  shifted  as  to  suggest  the 
appearance  of  new  bands,  but  Liveing  says  that  by  studying  solutions  of 
different  concentrations  he  has  convinced  himself  that  no  new  bands  ap- 
pear.    We  shall  see  that  this  is  an  error. 

Liveing  found  the  same  modifications  of  the  spectrum  in  aqueous  solu- 
tions produced  by  glycerol  as  by  alcohol.     Liveing*  concludes  thus: 

On  a  review  of  the  whole  series  of  observations,  I  conclude  that  the  characteristic 
absorptions  of  didymium  compounds,  namely  those  which  are  common  to  dilute  aqueous 
solutions,  and  are  only  modified  by  concentration,  by  heat,  and  by  variations  of  the  solvent, 
are  due  to  molecules  which  are  identical  in  all  cases,  though  their  vibrations  are  modified 
by  their  relations  to  other  molecules  surrounding  them. 

Urbain  ^  devised  a  new  method  for  separating  the  rare  earths,  using 
ethyl  sulphate. 

Drossbach,^  in  his  work  on  absorption  in  the  region  of  the  ultra-violet, 
measured  a  number  of  the  bands  of  praseodymium  and  neodymium;  and 
Hartley,*  in  his  work  on  the  absorption  spectra  of  metallic  nitrates,  included 
the  nitrate  of  erbium.  In  discussing  his  results,  Hartley  calls  attention  to 
the  fact  that  Bunsen^  found  that  didymium  salts  in  the  crystallized  state 
and  in  solution  show  absorption  bands  that  vary  in  width  with  the  thickness 
of  the  absorbing  medium  and  with  the  quantity  of  the  salt.  Solutions  of 
the  chloride,  sulphate,  and  acetate,  each  containing  the  same  weight  of  di- 
dymium, yielded  different  spectra,  the  bands  being  shifted  towards  the  red 
end  of  the  spectrum  with  increase  in  the  molecular  weight  of  the  salt. 

Hartley  calls  attention  to  the  fact  that  more  recently  Becquerel®  ob- 
served similar  variation  in  the  absorption  spectra,  both  in  crystals  and  in 
solutions,  while  Muthmann  and  StiitzeF  found  marked  differences  between 
the  spectra  of  solutions  of  the  different  salts  of  neodymium,  such  as  the  chlo- 
ride, nitrate,  and  carbonate.  As  Hartley  points  out,  these  facts  can  not  be 
reconciled  with  the  theory  that  the  absorption  spectra  of  solutions  of  neo- 
dymium salts  are  due  to  the  neodymium  ion,  since  the  solutions  of  the 
above-named  salts  contain,  for  comparable  concentrations,  practically  the 
same  number  of  neodymium  ions. 

Among  the  more  recent  investigations  made  upon  the  salts  of  the  rare 
earths  is  that  of  Miss  Helen  Schaeffer.^  She  attempted  to  test  Kundt's 
law  for  the  nitrates  of  certain  rare  earths  such  as  neodymium  and  cerium. 
She  employed  the  following  solvents  :  Water,  methyl  alcohol,  ethyl  alco- 
hol, propyl  alcohol,  isobutyl  alcohol,  amyl  alcohol,  allyl  alcohol,  glycerol, 
and   acetone.    Solutions  were   studied   containing   1  gram  of  the  salt  in 

» Camb.  Phil.  Soc,  18,  314  (1900). 

» Compt.  rend.,  126,  835,  127,  107  (1898). 

« Ber.  d.  deutsch.  chem.  Gesell.,  35,  I486  (1902).    Ann.  Chim.  Phys.  (7),  19, 184  (1900). 

*Joum.  Chem.  Soc,  83,  221  (1903). 

» Pogg.  Ann.,  128,  100  (1866). 

•Compt.  rend.,  104,  777,  1691  (1887). 

^  Ber.  d.  deutsch.  chem.  Gesell.,  32,  2653  (1899). 

» Phys.  Ztschr.,  7,  822  (1906). 


SALTS    OP    NEODYMIUM,    PRASEODYMIUM,  AND    ERBIUM.  71 

10  c.c.  of  the  solvent.  She  found  that,  in  general,  the  bands  had  a  differ- 
ent arrangement  in  the  various  solvents;  and  in  order  to  identify  the 
bands,  she  worked  with  mixtures  of  the  various  solvents,  so  as  to  get  what 
she  supposed  was  a  gradual  shift  of  the  bands.  Her  plates,  however,  show 
that  instead  of  having  a  shift  in  the  bands,  she  had  two  sets  of  bands 
existing  simultaneously. 

In  the  second  part  of  her  investigation  she  studied  change  in  absorp- 
tion with  change  in  concentration;  in  other  words,  Beer's  law.  She  found 
that  with  decreasing  concentration  there  is  a  shift  of  the  yellow  band 
towards  the  violet.  The  most  concentrated  solutions  with  which  she 
worked  contained  about  30.5  grams  of  didymium  nitrate  in  10  c.c.  water. 
She  concludes  that  all  of  the  facts  established  by  her  investigation  can  be 
accounted  for  in  terms  of  electrolytic  dissociation  alone. 

An  excellent  piece  of  work  on  the  absorption  spectra  of  aqueous  solu- 
tions of  neodymium  chloride  has  recently  been  done  by  Rech.^  The  ab- 
sorption bands  were  carefully  measured  as  far  down  into  the  red  as  the 
sensibility  of  his  plates  would  permit.  There  is  transmission  farther  down 
into  the  red  than  could  be  detected  by  the  plates  which  he  employed. 

The  absorption  spectra  of  a  number  of  the  powdered  salts  of  neo- 
dymium and  erbium  were  recently  studied  by  Anderson.*  These  included 
the  chloride,  nitrate,  sulphate,  and  oxalate.  He  found  that  each  salt  has  its 
own  definite  absorption,  which  is  different  from  that  shown  by  any  other  salt. 

PREPARATION   OF  ANHYDROUS   SALTS. 

When  working  in  non-aqueous  solvents,  it  is,  of  course,  necessary  to 
have  the  dissolved  salts  perfectly  anhydrous.  A  number  of  the  salts  used 
in  this  investigation  can  not  be  dried  in  the  air  by  simply  raising  the  tem- 
perature. Under  these  conditions  the  oxy-salt  would  be  formed.  This 
applies  to  most  of  the  chlorides  and  bromides,  whose  non-aqueous  solu- 
tions were  studied  in  this  work. 

In  every  such  case  the  chloride  in  question  was  dried  in  a  current  of  dry 
hydrochloric  acid  gas.  It  was  placed  in  a  porcelain  boat,  which  was  then 
inserted  into  a  glass  tube  through  which  a  current  of  dry  hydrochloric  acid 
gas  was  passed.  The  glass  tube  was  then  heated  in  an  air-bath  to  the  tem- 
perature required  to  remove  all  of  the  water  from  the  salt. 

In  removing  all  of  the  water  from  a  bromide,  the  salt  was  treated  in 
every  respect  like  the  chloride,  except  that  it  was  dried  in  a  current  of  dry 
hydrobromic  acid  gas.  The  usual  methods  for  testing  the  purity  of  all  of 
the  compounds  employed,  and  of  standardizing  the  mother-solutions  of 
these  substances,  were  used.  A  detailed  discussion  of  this  subject  would 
be  superfluous. 

The  praseodymium  and  neodymium,  in  the  form  of  the  double  nitrate 
with  ammonium,  were  furnished  us,  with  their  characteristic  generosity, 
by  the  Welsbach  Light  Company,  and  it  gives  us  unusual  pleasure  to 
express  here  our  heartiest  thanks  to  their  chemist,  Dr.  H.  S.  Miner.    The 

» Dissertation,  Bonn,  1906.  *  Astrophys.  Journ.,  26,  73  (1907). 


72  ABSOBPTION   SPECTRA   OF   SOLUTIONS. 

chemists  of  this  company  have  always  shown  a  spirit  of  cooperation  with 
scientific  work  on  the  rare  elements  that  is  very  unusual,  and  for  which 
men  of  science,  working  in  this  field,  owe  them  a  lasting  debt  of  gratitude. 
The  praseodymium  used  in  this  work  was  practically  free  from  neo- 
dymium,  containing  only  a  few  hundredths  of  1  per  cent.  From  the 
spectrograms  it  would  appear  that  the  neodymium  used  contained  about 
6  per  cent  of  praseodymium.  The  erbium,  of  course,  contained  quite  a 
considerable  amount  of  impurities. 

Neodymium  Chloride  in  Water — ^Beer's  Law.    (See  Plates  69,  60,  and  72  B.) 

Five  different  sets  of  solutions  were  made  up,  covering  as  wide  a  range 
of  concentrations  as  possible,  the  object  being  not  only  to  test  Beer's  law 
thoroughly,  but  also  to  get  as  complete  a  map  as  possible  of  the  absorption 
spectrum  of  neodymium  chloride.  In  very  concentrated  solutions  a  cer- 
tain group  of  bands  may  appear  as  a  single  band,  due  to  the  widening  of 
the  individual  bands  or  to  general  absorption  in  the  region  considered.  By 
diminishing  the  concentration  such  a  "band"  breaks  up  gradually  into  its 
components,  and  hence,  to  map  completely  the  absorption  spectrum,  it  is 
necessary  to  work  over  a  wide  range  of  concentrations. 

If  the  object  were  simply  to  "map  the  spectrum"  this  could,  of  course, 
be  most  conveniently  done  by  keeping  the  depth  of  layer  constant  and 
changing  the  concentration  through  a  sufficient  range,  thus  getting  the 
complete  spectrum  on  a  single  film;  but  since  the  chief  object  here  was  to 
test  Beer's  law  it  was  necessary  to  make  several  sets  of  solutions  covering 
different  ranges  of  concentration.  The  concentrations  of  the  solutions 
used  in  making  the  negative  for  A,  Plate  59,  beginning  with  the  one  whose 
spectrum  is  adjacent  to  the  numbered  scale,  were  3.40,  3.02,  2.72,  2.38,  2.17, 
1.90,  and  1.70;  the  corresponding  depths  of  cell  being  12,  13.5,  15,  17,  19, 
21.5,  and  24  mm.  For  B,  Plate  59,  the  concentrations  were  3.40,  2.55, 
1.70,  1.13,  0.80,  0.57,  and  0.43;  the  corresponding  depths  of  absorbing 
layer  being  3,  4,  6,  9,  13,  18,  and  24  mm.  For  A,  Plate  60,  the  concentra- 
tions were  1.70,  1.27,  0.85,  0.57,  0.40,  0.28,  and  0.22;  for  B,  Plate  60, 
they  were  0.85,  0.63,  0.42,  0.28,  0.20,  0.14,  and  0.11,  and  for  B,  Plate  72, 
0.42,  0.31,  0.21,  0.14,  0.10,  0.07,  and  0.055;  the  depths  of  absorbing  layer 
were  in  each  case  the  same  as  in  B,  Plate  59.  It  will  be  noticed  that  begin- 
ning with  B,  Plate  59,  the  concentrations  used  in  each  succeeding  set  are 
just  halved  each  time. 

The  most  concentrated  solutions  appeared  brownish-yellow  in  their 
bottles,  from  which  the  color  changed  on  dilution  to  a  yellowish-pink,  the 
color  being  extremely  faint  in  the  most  dilute  solutions. 

The  exposures  to  the  light  of  the  Nernst  lamp  and  spark  were,  respec- 
tively, 1  and  2  minutes,  the  slit  having  a  width  of  0.01  cm.  The  exposures 
and  slit  width  were  not  varied  in  the  work  recorded  in  the  present  chapter, 
the  object  being  to  make  the  spectrograms  as  nearly  comparable  as  possible. 

Both  A  and  B  of  Plate  59  show  the  presence  of  some  general  absorp- 
tion in  the  ultra-violet,  which  decreases  quite  rapidly  with  dilution.  The 
absorption  bands  also  narrow  somewhat  with  decrease  in  concentration, 
especially  from  3.4  normal  to  about  1.7  normal.    For  concentrations  less 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,  AND    ERBIUM. 


73 


than  about  1.5  normal  Beer's  law  seems  to  hold  very  accurately  indeed, 
with  the  exception  of  the  shading  towards  the  red  accompanying  the  band 
near  X  5800,  which  seems  to  decrease  somewhat  with  dilution  for  concen- 
trations of  normal  or  less. 

In  the  following  table  the  measurements  of  the  positions  of  the  bands 
were  made  on  the  seventh  strip  of  A,  Plate  59,  and,  therefore,  refer  to  a 
concentration  of  1.7  normal  with  a  depth  of  layer  of  24  mm.  The  remarks 
referring  to  changes  with  dilution  apply  to  a  change  in  concentration  from 
3.4  to  1.7  normal,  the  depths  of  layer  being  so  varied  that  the  product  of 
concentration  and  depth  remains  constant. 


A 

Character. 

Bemarks. 

2810 
2890-2910 
2970-2995 

3220-3330 

3380-3400 

3435-3595 
4180 
4275 
4290 

4330 
4410-4465 

4580-4650 

4665-4710 

4740-4770 

4820 
6000-5330 

5660-5930 
6235 

6260 
6270-6310 
6360-6390 

6730 
6770-6840 

6890 

7250 

Faint  transmission  begins. 
Band  with  well-defined  sharp  edges. 
A  double  band,  strongest  component  to 
violet. 

Strong    band    of   complete   absorption, 

sharp  edges. 
Rather     faint     band.       Most     intense 

towards  red. 
Complete  absorption.     Edges  sharp. 

The  observed  narrowing  with  dilution  per- 
haps due  largely  to  general  U.V.  absorp- 
tion. 

Narrows  slightly  with  dilution. 

Narrows  somewhat  with  dilution. 

Narrows  considerably  at  first. 

Between  this  and  A  4275  is  fairly  strong  ab- 
sorption in  the  most  concentrated  solution. 
This  absorption  has  disappeared  in  the 
spectrum  measured . 

This  band  is  coincident  with  band  due  to 
praseodymiimi,  and  is  to  be  ascribed  to 
this  element,  which  had  not  been  com- 
pletely separated  from  the  neodymium.  It 
does  not  change  with  dilution. 

Narrows  slightly  with  dilution. 

Partly   due    to   praseodymium.      Does    not 

change  with  dilution. 
Not  affected  by  dilution. 
Due  at  least  partly  to  praseodymium. 
Violet  shading  a  little  greater  in  concentrated 

solutions. 
Shading  on  red  side  decreases  with  dilution. 
Not  afifected  by  dilution, 

Do, 

Hazy. 

Edges  rather  hasy 

Band  with  hazy  edges,  not  completely 

separated  from  4665-4710. 
More  sharply  defined  on  red  than  on 

violet  side. 

Red  limit  sharp,  violet  a  little  hazy. . . . 

Violet  limit  sharp.     Red  edge  hazy 

First    and    strongest    band    in    orange 
group. 

Do. 

Faint  band 

Do. 

Faint.    In  shading  of  principal  red  band . 

Do, 

Do. 

Do. 

Do. 

The  most  marked  change  produced  by  dilution  from  3.4  to  1.7  normal, 
excepting  that  in  the  red  shading  of  the  X  5660  to  k  5930  band,  is  that  tak- 
ing place  on  the  red  side  of  the  narrow  absorption  line  at  X  4275.  In  the 
spectrum  of  the  most  concentrated  solution  the  red  edge  of  this  line  falls 
at  X  4280,  from  which  place  a  uniform  absorption  extends  to  X  4295.  In 
the  third  spectrum  counting  from  the  numbered  scale,  the  shading  has 
almost  completely  disappeared,  leaving  a  very  narrow  line  at  approximately 
X  4290.  The  width  of  this  line  is  only  2  or  3  A.U.,  and  it  persists  with 
unchanged  intensity  throughout  the  remaining  strips  of  the  spectrogram. 
Its  intensity  is,  however,  not  sufficient  to  make  it  show  in  the  reproduc- 
tion, and  not  even  great  enough  to  make  it  visible  on  the  negative  for  B, 
Plate  59. 


74  ABSOEPTION   SPECTRA  OF   SOLUTIONS. 

The  limits  of  transmission  for  the  yellow  band,  as  shown  by  the  spec- 
trum of  the  most  concentrated  solution,  are  A  5660  and  X  5950;  hence  the 
narrowing  of  its  red  side  amounts  to  20  A.U. 

B,  Plate  59,  starts  at  the  same  concentration  as  A,  but  the  effective 
depth  of  absorbing  layer  is  only  one-fourth  of  that  used  in  A.  Hence  this 
spectrogram  represents  the  spectrum  of  a  solution  of  neodymium  chloride 
24  mm.  deep  and  having  a  concentration  of  0.43  normal.  The  absorption 
bands  are  all  much  narrower,  and  several  of  them  are  shown  in  the  process 
of  breaking  up  into  simpler  bands.  The  bands  in  the  ultra-violet  have  dis- 
appeared, excepting  the  one  at  k  3435  and  X  3595,  which  is  still  intense,  and 
a  trace  of  the  one  at  A  3220  to  X  3330.  Transmission  in  this  region  now  ex- 
tends faintly  to  X  2460.    No  new  absorption  bands  beyond  X  2800  can  be  seen. 

The  X  3435  to  X  3595  band  now  has  the  limits  X  3450  to  X  3580,  and  shows  a 
weak  transmission  at  X  3485,  which  increases  somewhat  with  dilution,  thus 
dividing  the  band  into  two.  In  A,  Plate  60,  this  has  broken  up  further  into 
bands  having  their  centers  at  X  3465,  X  3500,  X  3540,  and  X  3560,  the  bands  at 
X  3465  and  X  3540  being  the  narrowest  and  most  intense.  In  B,  Plate  60, 
X  3465  and  X  3540  are  both  narrow,  intense  bands,  while  X  3500  is  faint  and 
wide;  X  3560  disappeared  entirely  as  a  band.  In  B,  Plate  72,  the  only  things 
that  remain  of  the  group  are  the  two  narrow  lines  at  X  3465  and  X  3540. 

The  band  at  X  4180  is  weak  throughout  B,  Plate  59,  and  may  be  said 
to  have  disappeared  in  A,  Plate  60.  The  narrow  band  at  X  4275  is  very 
persistent,  showing  as  a  fine  black  line  even  in  B,  Plate  72.  Its  width 
remains  about  the  same  as  that  shown  by  the  negative  for  B,  Plate  59, 
throughout  the  range  of  concentrations  studied.  The  band  at  X  4330  be- 
haves exactly  like  the  one  at  X  4180,  practically  disappearing  in  A,  Plate  60. 

The  band  having  its  middle  at  X  4445,  which  is  perhaps  entirely  due  to 
the  praseodymium  present  as  an  impurity,  may  be  seen  even  in  B,  Plate 
72,  although  it  is  weak  and  very  diffuse  there.  In  A,  Plate  59,  it  has  about 
the  same  intensity  as  it  shows  in  a  solution  of  praseodymium  chloride 
having  a  concentration  of  0.85  and  a  depth  of  absorbing  layer  of  3  mm. 
This  indicates  that  the  percentage  of  praseodymium  in  the  neodymium 
salts  used  was  about  6  per  cent.  The  band  at  X  4825,  partly  due  to  prase- 
odymium, may  also  be  seen  throughout  the  entire  series  under  considera- 
tion, the  wave-length  of  the  praseodymium  band  being  X  4815,  while  that 
of  the  band  showing  in  all  the  neodymium  spectra  has  the  position  X  4825, 
showing  that  neodymium  had  a  band  nearly  coincident  with  that  given  by 
praseodymium,  but  lying  a  little  closer  to  the  red  end  of  the  spectrum. 

The  remaining  praseodymium  band  has  the  position  X  4685,  thus 
nearly  coinciding  with  the  rather  narrow,  strong  neodymium  band  whose 
position  is  X  4695.  This  neodymium  band  shows  with  considerable  inten- 
sity even  in  Plate  72  B,  while  the  praseodymium  band  at  X  4685  is  so  much 
fainter  than  the  X  4445  band  due  to  the  same  substance  that  we  should 
hardly  expect  it  to  show  here. 

The  band  which  under  A,  Plate  59,  was  recorded  as  having  the  limits 
X  4580  to  X  4650  shows  in  B  as  a  hazy  band  with  its  center  at  X  4615,  to- 
gether with  a  narrow,  faint  line  at  X  4645.  The  band  is  visible  in  A,  Plate 
60,  but  has  practically  disappeared  from  view  in  B,  of  the  same  plate. 
The  narrow  line  at  X  4645  does  not  show  beyond  B,  Plate  59. 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM.  75 

The  band  which  in  the  table  is  recorded  as  A  4740  to  X  4770  has  in  B, 
Plate  59,  become  a  slightly  hazy  band  having  its  middle  at  X  4760.  Its 
intensity  is  intermediate  between  that  of  the  bands  at  X  4695  and  X  4825, 
and,  hence,  like  them  may  be  seen  faintly  even  in  B,  Plate  72. 

The  band  which  in  A,  Plate  59,  has  the  limits  X  5000  to  X  5330  breaks  up 
into  a  rather  complicated  series  of  bands  on  dilution,  some  idea  of  which 
may  perhaps  be  gained  from  the  following  :  B,  Plate  59,  shows  some  ab- 
sorption throughout  the  region  given,  but  with  a  deep,  narrow  band  at 
X  5090  and  faint  transmission  at  X  5100  and  in  the  region  X  5150  to  X  5180. 
Absorption  is  complete  from  X  5105  to  X  5150,  and  from  X  5180  to  X  5270. 
There  is  again  incomplete  absorption  from  X  5270  to  X  5330,  with  indica- 
tion of  a  band  at  A  5315.  In  A,  Plate  60,  the  general  shading  has  the  limits 
X  5050  to  X  5330,  and  it  shows  the  following  :  A  narrow  intense  band  at 
X  5090;  wide,  hazy  band  at  X  5125;  a  pair  of  very  intense,  narrow  bands 
at  X  5205  and  X  5222;  very  narrow  band  at  X  5255;  and  faint,  hazy  band  at 
X  5315.  B,  Plate  60,  shows  the  shading  diminished  very  much  in  intensity, 
and  all  the  bands  except  the  doublet  X  5205  to  X  5222  rather  faint.  The 
absorption  in  the  doublet  is  still  almost  complete.  B,  Plate  72,  still  shows 
the  doublet  very  strong,  the  remaining  absorption  bands  faint,  although 
still  visible. 

The  Hmits  of  the  yellow  band  in  B,  Plate  59,  are  X  5700  to  X  5880  in  the 
strip  corresponding  to  the  most  concentrated  solution.  The  band  narrows 
by  30  Angstrom  units  on  this  spectrogram,  the  narrowing  being  due  to  a 
decrease  in  the  shading  towards  the  red,  with  decrease  in  concentration. 
In  B,  Plate  60,  the  limits  of  the  band  are  X  5710  and  X  5840.  There  is  still 
considerable  shading,  but  it  decreases  only  very  slightly  with  dilution. 
The  band  begins  to  show  its  structure,  but  not  well  enough  to  allow  any 
measurements  to  be  made. 

In  B,  Plate  60,  and  B,  Plate  72,  the  band  has  broken  up  into  the  fol- 
lowing smaller  bands  :  X  5725,  narrow  and  moderately  intense.  X  5745 
and  X  5765,  double  band,  not  clearly  resolved,  the  red  component  being 
more  intense  than  the  violet.  This  double  band  is  the  most  intense  of  the 
group.  X  5795,  band  having  about  the  same  intensity  as  the  one  at  X  5725, 
but  being  much  wider  and  hazier.    X  5380,  very  faint  band. 

The  group  of  absorption  bands  in  the  orange,  given  in  the  table,  may 
be  seen  faintly  in  B,  Plate  59,  and  very  faintly  in  A,  Plate  60;  but,  like  the 
group  in  the  red  near  X  6800,  it  shows  no  further  breaking  up  into  more 
complicated  bands  on  dilution. 

B,  Plate  59,  shows  that  the  spectrum  ends  at  X  7310  in  what  appears 
to  be  an  absorption  band.  In  Plate  60,  and  B,  Plate  72,  it  is  seen  that 
there  is  a  very  intense,  narrow  band  at  X  7325;  another  narrow  but  fainter 
band  at  about  X  7350,  and  a  wide,  moderately  intense  band  at  X  7390  or 
X  7400,  beyond  which  the  plates  were  not  suJB&ciently  sensitive  to  give  any 
appreciable  photographic  action  with  the  length  of  exposure  used. 

The  most  intense  bands  of  neodymium  chloride,  and  hence  the  ones 
which  would  be  most  conspicuous  in  a  very  dilute  solution,  are  the  follow- 
ing :   X  3465,  X  3540,  X  4275,  X  5205,  X  5225,  X  5745,  X  5765,  and  X  7325. 

The  wave-lengths  of  all  the  bands  are  collected  in  the  following  table, 
together  with  a  brief  description  of  the  appearance  of  each  band.    It  is  to 


76 


ABSORPTION   SPECTRA   OF   SOLUTIONS. 


be  understood  that  this  table  is  not  meant  to  represent  what  could  be 
seen  or  photographed  in  any  one  solution  of  neodymium  chloride  in  water. 
It  merely  records  the  position  of  all  the  bands  which  can  be  seen  in  a  layer 
from  3  to  12  mm.  deep,  when  the  concentration  is  varied  from  0  to  3.4  normal. 


K 

Character. 

A 

Character. 

2900 

About  20  A.U.  wide. 

5205 

Very  intense,  narrow. 

2985 

About  25  A.U.  wide. 

6222 

Very  intense,  narrow. 

3225 

Narrow  and  sharp. 

6255 

Narrow,  intense. 

3390 

Narrow,  faint. 

6315 

Hazy  edges,  faint. 

3465 

Very  intense,  narrow. 
Rather  wide. 

6726 

Narrow,  intense. 

3505 

5745 

Very  intense. 

3540 

Very  intense,  narrow. 

6765 

Very  intense. 

3560 

Faint,  narrow. 

6795 

Intense,  moderately  narrow. 

4180 

Faint,  hazy. 

5830 

Very  faint  and  hazy. 

4275 

Very  intense  and  sharp. 

6235 

Fairly  narrow. 

4290 

Very  narrow,  faint. 

6260 

Very  narrow,  faint. 

4330 

Hazy  edges. 

6270-6310 

Faint,  hazy  edges. 

4615 

Rather  wide  and  hazy. 

6360-6390 

Faint,  hazy  edges. 

4646 

Very  narrow,  faint. 

6730 

Faint  band. 

4695 

Narrow,  intense. 

6800 

Moderately  intense,  hazy  edges. 

4760 

Hazy  edges,  fairly  narrow. 

6890 

Hazy  edges. 

4825 

Narrow  and  fairly  intense. 

7325 

Very  intense  and  narrow. 
Rather  wide  band. 

5090 

Narrow,  intense. 

7390 

6125 

Rather  wide  and  hazy. 

Neodtmium  Chloride  in  Water — ^Molecules  Constant.    (See  Plate  61.) 

The  dissociation  of  neodymium  salts  not  having  been  determined,  it 
was  assumed  that  their  dissociation  was  the  same  as  those  of  aluminium. 
Although  this  may  not  be  exactly  true,  yet  the  rate  of  change  of  dissocia- 
tion with  concentration  will  perhaps  be  practically  the  same  for  the  two 
metals,  and  that  is  the  only  thing  which  comes  into  account  here. 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  3.4,  2.7,  1.95,  1.44,  1.10,  0.86,  and  0.69;  the  corresponding  depths  of 
absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the  concentra- 
tions were  1.36,  1.10,  0.80,  0.59,  0.44,  0.35,  and  0.28;  the  depths  of  the 
absorbing  layers  were  the  same  as  in  A. 

Since  Beer's  law  holds  so  very  accurately  for  neodymium  chloride  in 
water,  excepting  at  the  very  greatest  concentrations,  it  is  to  be  expected 
that  when  molecules  are  kept  constant  all  the  bands  would  show  consider- 
able widening  with  dilution,  and  this  is  found  to  be  the  case.  It  will  be 
recalled,  however,  that  the  shading  on  the  red  side  of  the  yellow  band 
showed  considerable  deviations  from  Beer's  law,  even  at  moderate  dilu- 
tions; and  it  was  to  see  whether  there  is  any  connection  between  this 
shading  and  the  undissociated  molecules  that  the  present  spectrogram  was 
made.  Here  it  will  be  seen  that  the  shading  decreases  when  the  concentra- 
tion is  changed  from  3.4  to  1.95,  then  remains  sensibly  constant  until  the 
concentration  becomes  as  small  as  about  1.0,  when  it  increases  with  further 
dilution.  It  seems  evident,  then,  that  this  shading  can  not  be  ascribed  to 
the  undissociated  molecules,  any  more  than  can  the  rest  of  the  absorption 
phenomena  shown  by  these  solutions.  Apparently  the  absorption  depends 
only  upon  the  number  of  neodymium  atoms  present,  and  is  independent, 
or  nearly  so,  of  whether  these  exist  as  ions  or  combined  with  chlorine  to 
form  the  chloride  molecules. 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM.  77 

Neodymium  Chia)ride  in  Water  with  Calcium  Chloride  and  with 
ALTTMiNnjM  Chloride.    (See  Plate  62.) 

The  concentration  of  neodymium  chloride  in  all  the  solutions  was  the 
same,  namely  0.23.  The  concentrations  of  calcium  chloride,  beginning  with 
the  solution  adjacent  to  the  numbered  scale  of  A,  were  4.29,  3.68,  2.86,  2.29, 
1.72, 1.14, 0.57,  and  0.0;  the  corresponding  concentrations  of  aluminium  chlo- 
ride in  the  solutions  used  in  making  the  negative  for  B  being  2.80,  2.40, 2.00, 
1.60,  1.20,  0.80,  0.40,  0.00.     Depth  of  absorbing  layer  throughout,  2.0  cm. 

These  dehydrating  agents,  especially  the  aluminium  salt,  introduce  con- 
siderable general  absorption  in  the  ultra-violet.  This,  however,  is  due  to 
the  foreign  salt  itself,  and  is  in  no  way  to  be  ascribed  to  its  effect  on  the 
neodymium  salt  in  the  solutions.  This  general  absorption  also  accounts  for 
the  apparent  increase  in  intensity  of  the  ultra-violet  absorption  band  of 
neodymium  at  X  3500. 

The  shading  on  the  red  side  of  the  yellow  band  is  slightly  increased  by  the 
addition  of  calcium  chloride,  and  somewhat  more  so  by  the  addition  of  the  alu- 
minium salt.  Beyond  this  no  effect  on  the  absorption  spectrum  of  neodymium 
chloride  is  produced  by  even  large  quantities  of  these  dehydrating  agents. 

Neodymium  Chloride  in  Methyl  Alcohol — Beer's^Law.    (See  Plate  63.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.50,  0.40,  0.315,  0.25,  0.20,  0.16,  and  0.125;  the  correspond- 
ing depths  of  absorbing  layer  were  6,  7.5,  9.5,  12,  15,  19,  and  24  mm.  The 
concentrations  for  B  were,  in  the  same  order,  0.20,  0.16,  0.13,  0.10,  0.08, 
0.06,  and  0.05;  the  depths  of  cell  were  the  same  as  used  in  A. 

There  is  some  absorption  in  the  extreme  ultra-violet,  which,  however, 
is  to  be  ascribed  to  the  solvent  and  not  to  the  neodymium  chloride. 

No  trace  of  absorption  due  to  the  dissolved  substance  is  visible  until 
we  reach  the  group  of  bands  near  X  3500.  There  are  three  bands  having 
their  centers  at  X  3475,  X  3505,  and  X  3560.  Of  these  the  one  at  X  3560  is 
the  widest  and  also  the  most  intense;  the  one  at  X  3475  being  somewhat 
fainter  than  that  at  X  3505.  The  bands  are  all  much  wider  and  hazier 
than  those  occurring  near  the  same  place  in  the  aqueous  solution.  No 
change  with  dilution  indicating  a  deviation  from  Beer's  law  can  be  detected 
in  these  or  any  of  the  other  bands  in  the  alcoholic  solutions  of  the  chloride. 

In  the  violet  and  blue  regions  we  find  the  following  :  A  band  at  X  4290, 
about  10  A.U.  wide  and  only  moderately  intense.  At  X  4325,  a  band  some- 
what wider  and  fainter.  At  X  4460,  a  rather  wide  hazy  band  with  a  faint 
hazy  companion  towards  the  violet.  This  is  the  band  which  is  perhaps  due 
to  praseodymium.  The  much  greater  concentration  of  the  alcoholic  solu- 
tions of  praseodymium  chloride  studied  in  this  work  makes  it  impossible 
to  verify  this,  by  seeing  whether  the  praseodymium  band  in  dilute  solution 
really  has  this  general  character. 

There  are  bands  at  X  4700,  X  4780,  and  X  4825,  all  of  about  the  same 
intensity,  the  one  at  X  4770  being,  however,  much  narrower  than  the  other 
two,  of  which  X  4825  is  somewhat  the  wider.  Both  X  4700  and  X  4780  have 
faint  companions  to  the  violet. 


78  ABSORPTION   SPECTRA  OP   SOLUTIONS. 

The  group  in  the  green  is  made  up  of  six  bands  as  follows  :  X  5125,  hazy 
and  rather  wide,  moderately  intense;  X  5180,  also  hazy  but  much  fainter; 
k  5220,  moderately  intense  and  narrow;  }.  5245,  intense,  and  with  faint 
companion  towards  the  red;  X  5290,  narrow  and  moderately  intense;  shad- 
ing as  far  as  A  5330,  with  indications  of  a  faint  band  at  -^5315. 

The  yellow  group  is  made  up  of  seven  bands  having  the  following 
characteristics  :  X  5725,  moderately  intense  with  hazy  edges  ;  X  5765, 
narrower,  but  not  quite  as  intense  as  X  5725;  X  5800,  fairly  narrow,  strong; 
X  5835,  very  intense;  X  5860,  hazy  and  moderately  intense,  not  clearly 
separated  from  X  5835,  shading  to  X  5970,  with  two  faint  bands  super- 
imposed upon  it,  one  at  X  5895  and  the  other  at  X  5925. 

No  trace  of  bands  is  to  be  seen  in  the  orange,  but  in  the  red  there  is 
a  fairly  narrow  but  faint  band  at  X  6860.  The  spectrum  ends  at  X  7355 
in  a  deep,  rather  narrow  band.  It  is  evident  that  the  spectrum  of  neo- 
dymium  chloride  when  dissolved  in  methyl  alcohol  is  quite  different  from 
its  spectrum  in  aqueous  solution,  but  this  point  will  be  taken  up  more 
fully  in  the  discussion  of  Plates  65  and  66. 

Neodtmitjm  Chloride  in  ETHTii  Alcohol — Beer's  Law.    (See  Plate  64.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  0.50,  0.40,  0.315,  0.25,  0.20,  0.16,  and  0.125;  the  corresponding 
depths  of  absorbing  layer  being  6,  7.5,  9.5,  12,  15,  19,  and  24  mm.  For 
B,  the  concentrations  were  0.20,  0.16,  0.13,  0.10,  0.08,  0.06,  and  0.05;  the 
depths  of  cell  were  the  same  as  used  in  A.  The  concentrations  and  depths 
of  cell  were,  therefore,  exactly  the  same  as  those  in  methyl  alcohol,  so 
that  Plates  63  and  64  are  directly  comparable.  A  very  careful  comparison 
of  the  two  plates  reveals  the  remarkable  fact  that  the  two  spectra  are 
identical;  the  very  slight  differences  noted  being  perhaps  due  to  slight 
differences  in  development  of  the  negatives. 

In  view  of  the  great  difference  between  either  one  of  these  spectra 
and  that  of  the  aqueous  solution,  this  similarity  is  rather  surprising,  and  it 
led  us  to  think  that  perhaps  in  these  alcoholic  solutions  we  were  getting  the 
absorption  of  the  neodymium  chloride  molecules  themselves,  while  in  the 
aqueous  solution  we  get  the  absorption  of  some  compound  of  the  molecules 
with  water.  But  this  was  answered  in  the  negative  by  the  spectrum  of  anhy- 
drous neodymium  chloride  (Plate  68),  which  is  very  different  from  that  of  any 
of  the  solutions.  The  spectrum  of  the  alcoholic  solutions  is,  therefore,  not 
that  of  the  NdClg  molecule  per  se,  but  must  be  that  of  some  solvate  of  it  or  of 
the  neodymium  ion.  But  that  solvates  with  methyl  alcohol  and  ethyl  alco- 
hol should  affect  the  frequencies  of  the  vibrators  in  the  metallic  atom  so 
very  nearly  the  same  seems  a  little  surprising,  to  say  the  least,  especially  as 
solutions  of  the  nitrate  in  the  two  solvents  give  somewhat  different  spectra, 
as  will  be  fully  discussed  when  we  come  to  consider  Plates  73  and  74. 

The  very  slight  differences  between  the  bands  shown  by  Plates  64  and 
63  seemed  to  indicate  that  they  were  a  little  more  hazy  in  the  ethyl  alcohol 
solutions,  but  the  development  of  the  negatives  for  Plate  64  was  not  car- 
ried quite  as  far  as  was  the  case  with  those  for  Plate  63,  and  this  would 
tend  to  produce  just  the  kind  of  difference  that  was  noted. 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM.  79 

Neodyahum  Chloride  in  Mixtubes  of  Methyl  Alcohol  and  Wateb. 
(See  Plates  65,  66,  and  76  B.) 

Since,  as  we  have  just  seen,  the  absorption  spectrum  of  neodymium 
chloride  in  aqueous  solution  is  so  different  from  that  of  the  alcoholic  solu- 
tions, it  was  thought  to  be  of  some  interest  to  see  how  the  change  from  the 
one  to  the  other  would  take  place  if  one  of  the  solvents  was  made  to  dis- 
place the  other  gradually.  A  series  of  solutions  was  accordingly  made  up, 
the  concentration  of  the  dissolved  salt  being  constant  and  equal  to  0.5 
normal,  but  the  character  of  the  solvent  varying  as  follows  :  The  percent- 
ages of  water  in  the  seven  solutions  were  0,  16.6,  33.3,  50,  66.6,  83.3,  and 
100;  the  corresponding  percentages  of  methyl  alcohol  were  100,  83.3,  66.6, 
50,  33.3,  16.6,  and  0.  Two  spectrograms  were  made,  namely,  A,  Plate 
65,  where  the  depth  of  the  cell  was  1.5  cm.,  and  B,  where  the  cell  had  a 
depth  of  only  5  mm.  A  was  made  in  order  to  show  clearly  the  change 
taking  place  in  the  narrower  and  fainter  bands,  while  B  was  intended  to 
show  the  change  of  structure  of  the  more  intense  bands,  such  as  the  green 
and  yellow  ones.  The  strip  which  is  adjacent  to  the  numbered  scale  belongs 
to  the  solution  in  pure  water,  while  the  one  nearest  the  narrow,  comparison 
spark  spectrum  belongs  to  the  solution  in  pure  methyl  alcohol. 

Plate  65  shows  that,  beginning  with  the  strip  nearest  the  scale,  the  first 
six  spectra  are  very  nearly  identical.  From  the  sixth  to  the  seventh  there 
is  an  abrupt  change,  which  at  first  sight  consists  in  a  shift  of  all  the  bands 
towards  the  red,  but  which  on  closer  examination  is  seen  to  consist  in  a 
disappearance  of  one  spectrum  and  the  appearance  of  the  other.  Since  the 
first  strip  is  the  spectrum  of  the  solution  in  pure  water,  it  follows,  since  the 
sixth  is  nearly  identical  with  the  first,  that  as  large  a  percentage  of  alcohol 
in  the  solvent  as  83  per  cent  does  not  change  the  absorption  spectrum 
materially;  the  chief  change  taking  place  when  the  percentage  of  alcohol 
is  varied  from  83  per  cent  to  100  per  cent. 

It  is  to  be  noted  that  the  apparent  shift  of  the  bands  towards  the  red 
is  in  reality  not  quite  as  great  as  it  appears  at  first  sight  from  Plate  65, 
owing  to  the  fact  that  the  film  accidentally  shifted  slightly  towards  the 
red  between  the  sixth  and  seventh  exposures.  The  amount  of  this  me- 
chanical shift  is  easily  seen,  however,  by  comparing  the  spark  lines  in  the 
ultra-violet.  A  measurement  of  the  shift  shows  it  to  be  approximately  3 
Angstrom  units,  and  the  same  for  both  A  and  B,  while  the  "apparent" 
shift  of  the  absorption  line  at  X  4275  in  aqueous  solution  is  actually  15 
Angstrom  units,  its  position  in  the  alcoholic  solution  being  X  4290. 

The  slight  changes  taking  place  with  some  of  the  bands  throughout  the 
spectrograms  of  Plate  65  are  perhaps  sufficiently  clear  in  the  reproductions. 
However,  as  a  good  deal  of  the  detail  shown  by  the  negatives  is  lost,  even 
in  the  most  perfect  processes  of  reproduction,  we  give  here  a  description 
of  the  changes  taking  place  in  two  of  the  bands  as  seen  on  the  original 
negative.  We  select  the  bands  at  X  4275  and  X  4760  from  the  negative  for 
A,  Plate  65. 

In  the  aqueous  solution  the  A  4275  band  is  very  intense  and  narrow, 
its  whole  width  being  less  than  5  Angstrom  units.  The  edges  are  only 
very  slightly  shaded.    In  the  alcoholic  solution  the  position  of  the  center 


80  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

of  the  corresponding  band  is  X  4290.    It  has  a  width  of  from  12  to  13  Ang- 
strom units,  and  is  not  nearly  as  intense  as  in  the  aqueous  solution. 

Throughout  the  first  six  strips  the  X  4275  band  maintains  its  position 
and  intensity  almost  unchanged.  Its  position  does  not  change  in  the 
least,  but  its  intensity  in  the  sixth  strip  is  a  trifle  less  than  in  the  others. 
In  the  seventh  strip  there  is  not  the  faintest  trace  of  it  left.  In  the  third 
strip,  corresponding  to  the  solution  whose  alcohol  content  was  33.3  per 
cent,  there  appears  at  A  4285  an  extremely  faint  and  narrow  line.  In  the 
fourth  strip  it  is  somewhat  wider  and  more  intense,  but  its  center  is  still 
at  X  4285.  In  the  fifth  strip  it  is  beginning  to  be  fairly  conspicuous,  and 
in  the  sixth  it  is  a  band  of  moderate  intensity,  having  its  center  at  about 
X  4287.  This  band  is  undoubtedly  the  same  one  which  in  the  pure  alcoholic 
solution  has  its  center  at  X  4290  or  very  near  there,  the  exact  wave-length 
being  perhaps  nearer  to  X  4292.  We  see,  then,  that  even  when  the  mixed 
solvent  contains  only  about  one-half  alcohol,  this  band  exists  independent 
of  and  distinct  from  the  band  characteristic  of  the  aqueous  solution;  that 
it  is  at  first  only  a  very  narrow  and  faint  line,  which  widens  towards  the 
red  as  the  percentage  of  alcohol  is  increased. 

The  band  whose  center  is  at  X  4760  has  the  following  appearance  in  the 
aqueous  solution:  Faint  absorption  begins  at  X  4748  and  rises  rapidly  to 
a  maximum  between  X  4755  and  X  4760,  then  decreases  slowly  to  nothing 
at  X  4775.  The  band  is  accordingly  a  trifle  asymmetrical,  the  slope  towards 
the  violet  being  considerably  steeper  than  that  towards  the  red.  The 
corresponding  band  in  the  alcoholic  solution  is  double  and  answers  the 
following  description:  Very  faint  absorption  begins  at  X  4753,  and  rises  to  a 
faint  maximum  at  about  X  4757,  becoming  again  zero  at  X  4760.  It  begins 
again  at  X  4772,  rises  rapidly  to  a  strong  maximum  at  X  4780,  and  falls 
to  zero  at  X  4790.  The  component  whose  center  is  at  X  4757  is  very  faint 
compared  with  the  main  band. 

In  the  first  and  second  strips  we  have  nothing  but  the  band  correspond- 
ing to  the  aqueous  solution.  In  the  third  strip  the  red  side  of  the  band  has 
increased  slightly  in  intensity,  making  it  appear  much  more  nearly  sym- 
metrical. This  change  increases  in  the  fourth  and  fifth  strips,  the  band  at 
the  same  time  widening  considerably.  In  the  sixth  strip  its  appearance  is 
as  follows  :  Absorption  begins  at  X  4748  and  rises  to  a  maximum  just  to 
the  violet  side  of  X  4760,  then  decreases  slightly  towards  X  4770,  after  which 
it  increases  somewhat  to  X  4778,  then  falls  off  to  zero  at  X  4787.  It  is  very 
evident  from  a  study  of  the  change  in  this  band  that  the  two  hands  char- 
acteristic of  the  aqueous  and  alcoholic  solutions  coexist,  and  that  the 
band  appearing  in  our  photographic  strip  is  the  sum  of  the  two  taken  in 
different  proportions,  the  proportion  of  the  alcohol  band  being,  however, 
very  much  smaller  than  the  proportion  of  alcohol  in  the  corresponding 
solution.  A  similar  description  might  be  given  for  any  one  of  the  other 
bands,  but  this  is  not  necessary,  as  the  changes  are  of  exactly  the  same 
nature  as  those  we  have  already  indicated.  In  every  case  where  the  alco- 
holic solution  has  a  strong  band  which  differs  somewhat  in  position  from  any 
band  in  the  aqueous  solution,  we  begin  to  see  traces  of  this  band  when  the  pro- 
portion of  alcohol  in  the  mixture  reaches  50  per  cent;  but  the  band  remains 
comparatively  faint  even  when  the  proportion  is  as  high  as  83.3  per  cent. 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,    AND    ERBIUM.  81 

In  order  to  study  the  change  that  takes  place  between  the  sixth  and 
seventh  strips  of  the  spectrograms  of  Plate  65  more  carefully,  a  series  of 
alcoholic  solutions  was  prepared  containing  the  following  percentages  of 
water,  0,  2.6,  5.3,  8,  10.6,  13.3,  and  16.  The  concentration  of  the  neo- 
dymium  chloride  was  constant  and  equal  to  0.5  normal.  Two  spectrograms 
were  made,  one  with  a  depth  of  absorbing  layer  of  1.5  cm.  in  order  to  show 
the  fainter  bands,  and  the  other  with  the  depth  of  the  cell  only  5  mm.  in 
order  to  show  as  much  as  possible  of  the  structure  of  the  larger  bands. 
The  first  spectrogram  is  reproduced  as  Plate  66  A  and  the  second  as  Plate 
66  B.  The  strips  corresponding  to  the  pure  alcohol  solutions  are  adjacent 
to  the  numbered  scale,  the  spectrum  of  the  solution  containing  16  per  cent 
water  being  next  to  the  comparison  spark  spectrum. 

Although  we  found  on  considering  Plate  65  that  some  slight  change  in 
the  spectrum  takes  place  when  the  percentage  of  alcohol  is  changed  from 
0  to  83  per  cent,  yet  this  change  is  so  small,  and  the  bands  due  to  the 
aqueous  solution  are  so  strong,  that  we  may  regard  the  spectrum  of  a 
solution  containing  16  per  cent  of  water  as  practically  that  of  the  aqueous 
solution.  Accordingly,  the  spectrograms  on  Plate  66  may  be  taken  to  show 
very  nearly  the  whole  change  which  takes  place  when  the  solvent  of  neodym- 
ium  chloride  is  gradually  changed  from  pure  water  to  pure  methyl  alcohol. 

In  A  the  ultra-violet  band  is  rather  too  intense  to  allow  its  structure 
to  be  seen.  Accordingly,  we  see  the  whole  band  remain  sensibly  unchanged 
as  the  water  is  varied  from  16  per  cent  to  8  per  cent,  and  then  shift  towards 
the  red  with  increasing  rapidity  as  the  water  is  reduced  to  zero,  the  whole 
apparent  shift  amounting  to  about  20  Angstrom  units.  On  the  negative 
the  intense  band  at  X  3465  may,  however,  be  clearly  seen,  and  its  intensity 
decreases  very  slowly  from  the  first  to  the  third  strips,  counting  from  the 
narrow,  comparison  spark  spectrum.  In  the  fourth  strip  its  intensity  is 
about  half  of  what  it  was  in  the  first  strip,  and  from  this  it  decreases  rapidly, 
vanishing  entirely  in  the  strip  nearest  the  scale. 

In  B  the  structure  of  this  band  is  seen  very  distinctly,  and  we  find  that 
the  bands  characteristic  of  the  aqueous  solution  gradually  decrease  in 
intensity,  especially  from  the  third  to  the  sixth  strips,  while  the  wider 
bands,  characteristic  of  the  alcohohc  solutions,  increase  in  intensity,  the 
two  sets  existing  together.  The  change  in  the  band  at  X  4275  is  the  one 
that  shows  the  best,  because  here  the  two  bands  belonging  to  the  aque- 
ous and  alcoholic  solutions,  respectively,  are  both  intense  and  narrow  and 
clearly  separated  from  one  another. 

The  alcoholic  band  is  clearly  visible  in  the  first  strip,  and  it  increases 
continuously  in  intensity  as  the  amount  of  water  is  decreased,  but  more 
rapidly  from  the  fourth  to  the  seventh  strips  than  from  the  first  to  the 
fourth.  Its  position  also  shifts  somewhat  towards  the  red  from  the  first 
to  the  fourth  strips,  the  wave-lengths  of  its  center  for  the  two  strips  being, 
respectively,  X  4287  and  X  4292.  Accompanying  this  shift  is  a  change  in 
its  character,  which  may  be  gathered  from  the  following  statements  :  In 
the  first  strip  it  has  the  appearance  of  an  unsymmetrical  band,  the 
maximum  intensity  being  nearer  the  violet.  In  the  third  strip  it  extends 
from  X  4280  to  X  4295,  and  has  about  the  same  intensity  throughout.  In 
6 


82  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

the  fourth  strip  the  intensity  of  its  violet  edge  has  decreased  while  that  of 
the  red  edge  has  increased  considerably,  giving  it  the  appearance  of  an 
unsymmetrical  band,  with  the  maximum  intensity  towards  the  red.  In  the 
fifth  strip  the  violet  shading  from  X  4280  to  about  X  4284  has  disappeared, 
leaving  a  band  very  nearly  symmetrical  about  X  4290.  It  appears,  there- 
fore, that  we  are  really  dealing  with  two  unresolved  bands,  one  having  its 
center  at  about  X  4285  and  the  other  at  X  4292. 

The  band  at  X  4275,  due  to  the  aqueous  solution,  decreases  in  intensity 
throughout,  but  more  rapidly  from  the  third  to  the  sixth  strips  than  at 
first.  Its  position  remains  the  same  throughout.  As  near  as  the  eye  can 
judge  this  band  has  had  its  intensity  reduced  to  about  half  value,  when 
the  fourth  strip  is  reached,  corresponding  to  8  per  cent  of  water  in  the 
solution.  The  alcohol  band  at  X  4292  also  has  about  50  per  cent  of  its  final 
intensity  in  the  same  solution. 

The  band  at  X  4760  shows  the  same  kind  of  a  change  that  we  described 
in  some  detail  above,  only  here  the  change  is  much  more  gradual  and  easy 
to  follow.  It  also  shows  about  equal  intensity  for  the  two  sets  of  bands 
when  the  amount  of  water  is  8  per  cent  of  the  whole. 

The  green  and  yellow  bands  are  not  sufficiently  resolved  in  A  to  allow 
the  change  in  the  individual  bands  to  be  followed,  and  hence  these  ap- 
parently show  only  a  gradual  shift  towards  the  red  with  decrease  in  the 
amount  of  water.  In  B,  however,  they  are  both  sufficiently  resolved  to 
enable  us  to  follow  the  change  in  each  individual  band,  which,  although 
a  little  difficult  on  account  of  their  large  number  and  the  incompleteness 
of  their  separation,  in  some  cases  may  still  be  done.  The  change  is  in 
every  respect  the  same  as  we  have  found  for  the  other  bands,  namely, 
those  due  to  the  aqueous  solution  diminish  in  intensity,  and  reach  about 
half  value  in  the  8  per  cent  water  solution,  while  those  belonging  to  the 
alcoholic  solution  increase  in  intensity  as  the  amount  of  water  is  decreased. 

The  band  in  the  red  near  X  6800  shows  the  change  very  well  indeed, 
the  "water"  band  having  the  position  X  6800,  while  that  pertaining  to  the 
alcoholic  solution  is  situated  at  X  6860,  and  hence  the  two  are  well  sepa- 
rated. Here  the  point  of  equal  intensity  appears  to  be  reached  in  the 
solution  containing  10.6  per  cent  of  water,  but  this  is  due  to  the  fact  that 
the  alcoholic  band  has  a  considerably  greater  intensity  than  that  due  to 
the  aqueous  solution,  conditions  as  to  concentration  and  depth  of  layer 
being  the  same.  Taking  this  into  account,  it  is  seen  that  this  band  obeys 
substantially  the  same  rule  as  the  others. 

The  change  in  the  band  at  X  7325  is  more  difficult  to  follow  on  account 
of  the  small  intensity  of  the  photographic  action  on  the  less  refrangible  side 
of  this  position.  The  band  belonging  to  the  aqueous  solution  may  be  seen 
very  clearly,  even  in  the  strip  corresponding  to  the  2.6  per  cent  water  solution, 
but  is,  of  course,  entirely  absent  in  the  alcoholic  solution.  Its  intensity  in 
the  2  per  cent  solution,  however,  seems  a  little  greater  than  we  should  expect 
from  the  behavior  of  the  other  bands,  but  this  is  perhaps  due  to  the  rather 
weak  photographic  action  in  this  part  of  the  spectrum,  combined  with  the 
great  intrinsic  intensity  of  the  band.  The  alcoholic  solution  transmits  light 
as  far  as  X  7355,  where  its  spectrum  ends  abruptly  in  a  band. 


# 
SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM.  83 

Throughout  this  description  we  have  laid  great  stress  on  the  fact  that 
on  Plate  66  the  two  sets  of  hands  coexist,  the  bands  due  to  the  aqueous 
solution  decreasing,  while  those  belonging  to  the  alcoholic  solution  increase 
in  intensity  with  decrease  in  the  'percentage  of  water;  we  have  also  called 
attention  to  the  fact  that  the  two  sets  of  bands  have  about  half  their  full 
intensity  in  a  solution  containing  about  8  per  cent  of  water.  This  was  for 
a  0.5  normal  solution. 

The  next  question  which  suggested  itself  was  whether  the  composition 
of  the  solvent,  in  order  to  give  the  two  sets  of  bands  with  about  half  their 
normal  intensity,  is  independent  of  the  concentration  of  the  dissolved 
substance.  If  this  be  independent  of  the  concentration,  then  we  should 
have  to  conclude  that  the  determining  factor  is  the  nature  of  the  solvent; 
while  if  it  depends  upon  the  concentration,  the  ratio  between  the  amount 
of  dissolved  substance  and  one  or  other  of  the  solvents  would  perhaps  be 
the  important  thing.  To  answer  this  question  a  set  of  solutions  was  made 
up,  keeping  the  solvent  exactly  the  same  as  it  was  for  the  solutions  used 
in  making  the  negatives  for  Plate  66,  but  making  the  concentration  of 
neodymium  chloride  0.25  normal  instead  of  0.5  normal.  The  resulting 
spectrogram  is  shown  in  Plate  76  B.  In  order  to  have  this  spectrogram 
directly  comparable  with  B,  Plate  66,  the  depth  of  cell  was  kept  at  1.0 
cm.  throughout. 

A  study  of  this  negative  shows  that  the  two  sets  of  bands  have  about 
half  their  normal  intensity  in  the  third  strip,  counting  from  the  numbered 
scale,  corresponding  to  5.3  per  cent  of  water.  In  the  fourth  strip  the  bands 
characteristic  of  the  alcoholic  solutions  are  very  weak  compared  with  the 
bands  belonging  to  the  aqueous  solution,  while  in  the  second  strip  the 
opposite  is  the  case.  It  is  plain,  therefore,  that  the  composition  of  the 
solvent,  in  order  that  the  two  sets  of  bands  may  show  with  about  half  their 
normal  intensity,  depends  upon  the  concentration,  and  it  also  seems  very 
probable  that,  provided  the  ratio  of  water  to  dissolved  substance  is  kept 
constant,  the  two  sets  of  bands  will  not  vary  much  in  relative  intensity. 
A  simple  calculation  shows  that  in  the  solutions  which  produced  the  bands 
with  about  half  their  normal  intensity,  there  were  present  approximately 
10  molecules  of  water  to  1  molecule  of  neodymium  chloride. 

Neodtmium  Chloride  in  Ethyl  Alcohol  with  Water,    (See  Plate  67  A.) 

The  concentration  of  neodymium  chloride  was  constant  and  equal  to 
0.5  normal.  The  percentages  of  water,  beginning  with  the  solution  whose 
spectrum  is  adjacent  to  the  numbered  scale,  were  0,  5.3,  10.6,  16,  21.3,  26.6, 
and  32.    The  depth  of  the  cell  throughout  was  0.5  cm. 

This  spectrogram  shows  exactly  the  same  kind  of  change  that  we  have 
considered  rather  fully  under  Plate  66.  The  increments  of  water  added 
were  twice  as  large  here  as  in  the  case  of  methyl  alcohol,  and  hence  the 
change  takes  place  more  rapidly  as  we  pass  from  strip  to  strip,  beginning 
with  the  one  next  to  the  numbered  scale.  It  is  seen  that  in  the  second 
strip  the  bands  characteristic  of  the  alcohol  solution  are  very  much  more 
prominent  than  those  belonging  to  the  water  solution,  while  in  the  third 
strip  the  reverse  is  true.    This  points  to  the  fact  that  here  too  the  com- 


84  ^      ABSORPTION  SPECTRA  OF  SOLUTIONS. 

position  of  the  solvent,  in  order  to  give  the  bands  with  about  half  their 
normal  intensity,  would  be  7  or  8  per  cent  water  and  the  rest  alcohol. 
In  other  words,  we  again  find  complete  agreement  between  solutions  of 
neodymium  chloride  in  the  two  alcohols. 

If  the  fact  described  under  the  last  heading,  that  the  relative  inten- 
sities of  the  two  sets  of  bands  depend  only  upon  the  ratio  of  water  to 
neodymium  chloride  in  solution,  should  be  found  to  hold  even  for  concentra- 
tions of  one-tenth  or  one-hundredth  of  those  employed  here,  this  ought  to 
furnish  a  very  convenient  optical  method  of  detecting  rather  small  quan- 
tities of  water  in  alcohol ;  for  it  is  apparent  that  with  a  quarter  normal 
solution,  1  per  cent  of  water  gives  the  bands  due  to  the  aqueous  solution 
with  sufficient  intensity  to  be  seen  easily  with  a  small  spectroscope  if  a 
layer  of  a  centimeter  or  so  in  depth  is  used.  Accordingly,  to  detect  an 
amount  of  water  as  small  as  0.01  per  cent,  it  would  only  be  necessary  to  dis- 
solve in  the  alcohol  enough  anhydrous  neodymium  chloride  to  make  a  -^ 
normal  solution,  and  fill  a  glass  tube  with  the  solution,  so  as  to  get  a  layer 
from  50  to  100  cm.  deep,  when  the  bands  due  to  water  should  easily  be  seen. 

Neodymium  Chlobide — Anhydhous.    (See  Plate  68.) 

This  plate  was  made  in  order  to  see  whether  the  spectrum  of  the  an- 
hydrous salt  is  identical  with  that  observed  when  the  salt  is  dissolved  in 
pure  methyl  or  ethyl  alcohol.  The  anhydrous  salt  was  in  the  form  of  a 
very  fine  powder,  and  contained  in  a  bottle  with  a  tight-fitting  glass  stopper. 
An  image  of  the  Nernst  filament  was  thrown  on  the  surface  of  the  powder 
in  contact  with  the  walls  of  the  bottle,  and  this  image  was  in  turn  focussed 
on  the  sht  of  the  spectroscope  by  means  of  the  concave  spectrum  mirror. 
The  light  falling  on  the  grating  was  necessarily  very  faint;  therefore,  rather 
long  exposures  were  necessary;  but  this  caused  no  inconvenience,  since 
the  Nernst  lamp  burns  so  steadily  that  it  needed  no  attention  whatever. 
In  order  to  show  as  well  as  possible  both  the  strong  and  the  weak  bands, 
a  series  of  exposures  were  made  on  the  same  film,  the  times  of  exposure, 
beginning  with  the  strip  nearest  the  numbered  scale,  being  30  minutes, 
1  hour,  IJ  hours,  2  hours,  and  2^  hours.  On  account  of  the  fact  that  the 
beam  of  light  had  to  pass  through  the  glass  condensing  lenses,  as  well  as 
the  glass  walls  of  the  containing  bottle,  the  spectrum  ends  at  about  A  3450 
for  the  strip  nearest  the  comparison  spectrum,  and  at  X  3600  for  the  one 
nearest  the  scale. 

The  comparison  spark  spectrum  in  this  case  was  made  by  using  zinc 
terminals  instead  of  the  carbon  terminals  employed  throughout  the  rest 
of  the  work.  Since  there  is  usually  some  accidental  shift  between  the 
successive  strips  on  a  film,  and  since  no  light  but  that  of  the  Nernst  fila- 
ment was  used  in  making  the  five  strips  on  Plate  68,  it  is  evident  that  no 
accurate  wave-length  measurements  could  be  made  by  a  comparison  with 
the  spark  spectrum  on  this  plate.  In  fact,  the  position  of  a  given  absorp- 
tion line,  which  appeared  both  on  the  film  and  on  the  red-sensitive  plate, 
was  found  to  differ  by  as  much  as  10  Angstrom  units  as  measured  from  the 
two  negatives.  Hence  it  was  necessary  to  determine  the  position  of  one 
or  more  of  the  absorption  lines  by  comparison  with  a  spark  spectrum  which 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM. 


85 


should  have  been  impressed  on  the  plate  without  moving  this  between 
the  exposures  to  the  light  reflected  from  the  chloride  powder  and  to  that 
from  the  zinc  spark.  This  was  accomplished  by  making  an  exposure  of 
about  an  hour  for  the  absorption  spectrum  and  then,  without  moving  the 
plate  holder,  impressing  the  ultra-violet  portion  of  the  spark  spectrum  on 
the  same  strip.  Thus,  the  position  of  a  few  of  the  strongest  and  sharpest 
absorption  bands  was  determined,  and  the  positions  of  the  others  were 
measured  by  determining  their  distances  from  the  standards. 

On  the  whole,  the  spectrum  is  similar  to  that  observed  in  solutions; 
that  is,  if  the  solutions  show  a  group  of  absorption  bands  in  a  certain 
region,  then  there  is  also  a  group  of  bands  in  nearly  the  same  place  in  the 
spectrum  of  the  light  reflected  from  the  anhydrous  salt ;  but  as  a  rule  the 
individual  bands  in  the  group  are  much  narrower  and  more  numerous  in 
the  latter  than  in  the  former.  This  agrees  with  what  has  previously  been 
found  by  Becquerel  *  and  by  one  of  us.' 

In  the  following  table,  the  position  and  character  of  the  stronger  bands 
are  given.  No  attention  was  paid  to  the  numerous  bands  that  are  so 
faint  as  to  require  special  precautions  in  order  to  study  them,  as  the  object 
of  the  present  work  was  not  so  much  the  cataloguing  of  the  spectra  as  to 
try  to  get  some  idea  of  the  causes  of  the  changes  which  take  place  when  the 
substance  is  subjected  to  different  conditions. 


\ 

Character. 

A 

Character. 

3500 

Rather  strong,  narrow  band. 

5183 

Not  as  narrow  as  6174. 

3637 

Weaker  and  wider. 

6216 

Shaded  to  violet. 

3670 

Narrow  and  intense. 

6264 

Very  intense  and  narrow. 

3695 

Narrow  and  very  intense. 
Rather  faint  and  hazy. 

5267 

Very  intense  and  narrow. 

3612 

6282 

Weaker  and  wider  than  the  last  two. 

4045 

Weak  and  hazy.    Perhaps  2  or  3  bands. 
More  intense,  but  hazy. 

6300 

Shaded  towards  red,  perhaps  double. 

4080 

6328 

Intense,  narrow. 

4210 

Faint,  narrow. 

6342 

Weaker  and  broader. 

4228 

Faint,  perhaps  2  bands. 

6760-6000 

Strong  general  absorption. 

4308 

Very  narrow  and  intense. 

6768-5782 

Very  intense,  double  band. 

4313 

Very  narrow  and  intense. 

6807 

Narrow  and  intense. 

4333 

Wider  and  a  little  hazy,  but  intense. 

5829 

Most  intense  band  in  spectrum. 

4357 

Narrow,  shaded  towards  red. 

5858 

Very  narrow. 

4455 

Wide  and  hazy. 

6875 

A  little  hazy. 

4500 

Wide  and  hazy. 

5890 

Weak, 

4640 

Faint,  hazy. 

6902 

Fairly  narrow  and  intense. 

4680 

Faint,  hazy. 

6922 

Hazy  and  faint. 

4717 

Narrow  and  moderately  intense. 

6946 

Narrow,  intense. 

4725 

Narrow  and  moderately  intense. 

6968 

Wide,  faint  and  hazy. 

4735 

Narrow  and  moderately  intense. 

6265 

Wide,  moderately  intense. 

4775-4790 

Sharp  on  violet  side,  perhaps  2  bands. 

6290 

Narrow,  faint. 

4815 

Rather  narrow. 

6325 

Narrow,  faint. 

4855 

Intense  and  narrow. 

6375 

Narrow,  faint. 

4872 

Weak. 

6775 

Wide,  faint. 

4888 

Narrow,  moderately  intense. 

6796 

Narrow,  faint. 

4895 

Narrow,  moderately  intense. 

6816 

Narrow,  faint. 

6000-6370 

Strong  general  absorption. 

6838 

Moderately  intense. 

6088 

Weak,  slightly  hazy. 

6860-6900 

Band,  shading  towards  red. 

5117 

Stronger,  shaded  somewhat. 

6922 

Moderately  intense. 

5147 

Narrow,  intense,  hazy  on  violet  edge. 

7422 

Narrow,  intense  band. 

5174 

Intense,  slightly  hazy. 

It  is,  of  course,  evident  that  the  spectrum  of  the  solutions  of  neodym- 
ium  chloride  dissolved  in  methyl  or  ethyl  alcohol  is  very  far  from  being 
that  of  the  anhydrous  salt.    It  seems  reasonable  to  suppose  that  if  the 


*  H,  Becquerel,  Ann,  Chim,  Phys,  (6),  14,  pp.  257  et  seq, 

'  J.  A.  Anderson,  Astrophys.  Joum.,  26,  Sept.,  1907,  pp.  73-94, 


86  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

molecules  of  the  salt  in  the  non-aqueous  solutions  exist  in  the  free  state, 
that  is,  not  combined  with  the  solvent  in  any  way,  they  should  give  about 
the  same  spectrum  as  they  do  when  in  the  state  of  the  dry  powder.  Since 
they  do  not  do  this,  we  must  suppose  thai  the  solvent  plays  an  important  rdle 
in  determining  the  character  of  the  absorption,  and  how  it  can  do  this  with- 
out being  combined  with  the  salt  in  some  way  is  not  easy  to  understand. 

Neodymium  Bromide  in  Water — Beer's  Law.    (See  Plate  69.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.3,  1.7,  1.15,  0.77,  0.54,  0.38,  and  0.29;  the  corresponding 
depths  of  absorbing  layer  were  3,  4,  6,  9,  13,  18,  and  24  mm.  For  B  the 
concentrations  were  0.57,  0.42,  0.29,  0.19,  0.13,  0.09,  and  0.07;  the  depths 
of  the  absorbing  layer  were  the  same  as  in  A. 

The  bromide  solutions  are  very  much  redder  in  color  than  those  of  the 
chloride  or  nitrate.  Judging  from  the  color  alone,  one  would  say  that  the 
nitrate  solutions  are  much  more  transparent  in  the  blue  and  violet  than  the 
chloride,  and  the  chloride  solutions  much  more  so  than  those  of  the  bromide. 
The  spectrograms  do  not  show  this,  at  least  not  very  clearly;  which  merely 
indicates  that  where  full  exposures  are  given,  slight  general  absorption  is 
not  recorded  by  the  photographic  plate.  A  spectrophotometric  compari- 
son of  the  light  transmitted  through  these  solutions,  such  as  is  now  in 
progress  in  the  present  work,  will  undoubtedly  show  this  general  absorption 
of  the  bromide  solutions  in  the  more  refrangible  portion  of  the  spectrum. 

In  studying  the  spectrograms  of  this  plate,  A  was  compared  with  Plate 
59  B,  and  B  with  Plate  60  B,  that  is,  the  spectrum  of  the  bromide  solu- 
tions was  compared  with  that  of  a  chloride  solution  whose  concentration, 
in  each  case,  was  almost  exactly  1.5  times  that  of  the  bromide  solution, 
the  depth  of  the  absorbing  layer  being  the  same  in  both  cases.  The  two 
spectra  were  found  to  be  almost  identical,  except  in  the  extreme  ultra- 
violet, where  the  bromide  solutions  absorb  much  more  strongly.  The 
limits  of  transmission  for  the  most  concentrated  and  most  dilute  solu- 
tions of  A  are,  respectively,  A  3270  and  X  3050;  whereas  the  correspond- 
ing chloride  solutions  transmitted  to  beyond  X  2500.  The  ultra-violet 
absorption  shown  by  B  is  about  the  same  as  that  of  the  chloride  solutions 
used  in  making  Plate  59  B. 

The  absorption  bands  have  in  general  about  the  same  intensity  and 
character  in  the  bromide  solutions  as  they  have  in  the  corresponding 
solutions  of  the  chloride,  indicating  a  considerably  greater  absorbing  power 
of  the  bromide,  since  the  concentrations  of  its  solutions  were  only  0.66 
of  that  of  the  chloride.  A  small  part  of  this  is  due  to  the  fact  that  the 
negatives  for  Plate  69  were  not  as  fully  developed  as  those  made  with  the 
chloride  solutions,  but  even  if  the  development  had  been  exactly  the  same, 
the  bands  of  the  bromide  solutions  would  only  have  been  very  slightly 
less  intense  than  those  of  the  chloride  solutions.  We  must,  therefore,  con- 
clude that  in  solutions  of  the  same  concentration  the  bands  of  the  chloride 
solution  would  have  only  about  75  per  cent  of  the  intensity  of  the  same 
bands  in  the  spectrum  of  the  solution  of  the  bromide. 


* 

SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM.  87 

It  will  be  remembered  that  the  very  concentrated  solutions  of  the 
chloride  showed  some  slight  deviations  from  Beer's  law,  the  absorption 
to  the  red  side  of  the  narrow  band  at  X  4275  being  described  in  some  detail. 
The  deviations  from  Beer's  law  are  smaller  in  the  bromide  solutions,  per- 
haps on  account  of  the  concentrations  being  less.  No  shading  or  fine  ab- 
sorption line  between  A  4275  and  A  4290  is  to  be  seen  in  the  spectra  of  even 
the  most  concentrated  solutions  used  in  making  the  negative  for  A  of 
Plate  69.  The  shading  on  the  red  side  of  the  yellow  band  narrows  some- 
what with  increasing  dilution,  but  not  quite  as  rapidly  as  was  the  case 
with  the  chloride. 

Some  neodymium  bromide  was  dehydrated  in  a  current  of  hydrobromic 
acid  and  dissolved  in  methyl  alcohol,  and  also  in  mixtures  of  methyl  alcohol 
and  water.  The  solution  in  methyl  alcohol  was  stable,  and  showed  the 
same  spectrum  as  a  solution  of  the  chloride  in  the  same  solvent.  On  add- 
ing water,  precipitates  were  formed,  indicating  some  chemical  change. 
These  were  filtered  out,  and  a  spectrogram  made  to  see  whether  the  same 
changes  take  place  in  this  case  that  we  observed  with  the  chloride.  This 
spectrogram  is  not  reproduced,  but  it  indicated  that  the  changes  which 
took  place  were  quantitatively  as  well  as  qualitatively  the  same  as  those 
which  we  discussed  under  Plates  65  and  66. 

Neodymium  Nitkate  in  Water — Beer's  Law.    (See  Plates  70  and  71.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 

A,  Plate  70,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the 
numbered  scale,  were  2.96,  2.22,  1.48,  0.99,  0.69,  0.50,  and  0.38.  For  B 
the  concentrations  were  1.48,  1.11,  0.74,  0.50,  0.35,  0.25,  and  0.19.  For  A, 
Plate  71,  they  were  0.74,  0.55,  0.37,  0.25,  0.175,  0.125,  and  0.095;  and  for 

B,  0.37,  0.275,  0.185,  0.125,  0.092,  0.062,  and  0.048.  The  depths  of  absorb- 
ing layer  were  in  each  case  3,  4,  6,  9,  13,  18,  and  24  mm. 

The  nitrate  solutions  are  much  less  yellow  than  the  chloride  solutions, 
having  when  concentrated  a  decided  pinkish  tint,  indicating  greater  trans- 
parency in  the  violet  region  of  the  spectrum. 

The  spectrum  of  the  nitrate  solutions,  especially  when  the  concentra- 
tion is  considerable,  differs  quite  a  little  from  that  of  the  chloride.  It  is 
true  that  at  first  glance  they  seem  identical,  for  wherever  there  is  a  band 
in  the  spectrum  of  the  chloride  solution  a  band  is  found  when  the  nitrate 
solution  is  examined;  but,  at  least  in  concentrated  solutions,  the  bands 
have  a  very  different  appearance.  The  general  difference  is  that  the  nitrate 
bands  are  much  broader  and  hazier  than  those  observed  with  the  chloride. 
With  dilution  the  spectrum  of  the  nitrate  changes  very  much  more  than 
that  of  the  chloride,  which  we  found  practically  unaltered  when  the  concen- 
trations were  changed  from  about  1.5  normal  nearly  to  zero.  The  spectrum 
of  the  nitrate  solutions  changes  somewhat,  even  in  B,  Plate  71,  where  the 
concentration  ranges  from  0.37  to  0.048  normal. 

Instead  of  giving  a  detailed  description  of  the  spectrum  of  the  nitrate, 
we  will  limit  ourselves  to  a  description  of  the  changes  that  take  place  in 
a  few  of  the  bands,  which  differ  most  from  the  corresponding  bands  in  the 
spectrum  of  the  chloride  solution. 


88  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

Let  us  consider  first  the  band  at  A  4275.  In  the  spectrum  of  the  chloride 
solution  this  band  has  the  width  of  only  a  few  Angstrom  units  and  is  very- 
intense.  In  the  most  concentrated  nitrate  solution  this  band  has  a  width 
of  15  A.U.  and  its  center  falls  at  about  X  4280.  Its  edges  are  rather  hazy, 
but  the  band  is  very  symmetrical.  With  increasing  dilution  the  violet 
edge  increases  in  intensity,  taking  more  and  more  the  form  of  a  narrow 
absorption  line  with  center  at  A  4275;  while  the  red  portion  of  the  band 
decreases  in  intensity,  and  at  a  concentration  of  0.38  and  a  depth  of  layer 
of  24  mm.  it  has  taken  the  form  of  a  slightly  hazy  band  with  its  center 
near  X  4282.  This  band  is  here  clearly  separated  from  the  more  intense 
and  narrower  one  at  A  4275.  With  a  concentration  of  0.19  normal  and  a 
layer  24  mm.  deep,  the  X  4282  band  has  become  a  mere  shade  on  the  red 
side  of  the  X  4275  band;  and  finally,  with  a  concentration  of  0.048  and  a 
24  mm.  layer  of  the  solution,  it  is  no  longer  visible  on  the  photograph. 

At  X  4330  or  X  4335  the  concentrated  chloride  solutions  show  a  rather 
wide  hazy  band,  the  intensity  of  which  is  not  sufficient  to  allow  it  to  be 
seen  in  solutions  of  less  than  1.0  normal  with  a  depth  of  layer  of  5  mm.  or 
less.  The  more  dilute  solutions  of  the  nitrate  used  in  making  A,  Plate  70, 
show  this  band  with  about  the  same  intensity  and  character  that  it  has 
in  solutions  of  the  chloride;  while  the  very  concentrated  nitrate  solutions 
show  it  very  faintly;  that  is,  the  band  increases  in  intensity  with  dilution. 
In  fact  it  behaves  very  much  like  the  X  4275  band,  indicating  that  the  two 
owe  their  origin  to  the  same  "absorber." 

The  3.4  normal  chloride  solution  in  a  layer  3  mm.  deep,  shows  a  band 
at  X  4760,  to  which  the  following  description  applies  :  Absorption  begins 
at  X  4750,  rises  gradually  to  a  maximum  at  X  4760,  then  gradually  falls 
to  zero  at  X  4770.  This  band  remains  practically  constant  throughout  the 
series  of  solutions  used  in  making  B,  Plate  59,  showing  that  it  is  practically 
unaffected  by  change  in  concentration. 

The  2.96  normal  solution  of  the  nitrate,  with  a  layer  3  mm.  deep,  shows 
a  band  in  the  same  region  which  has  the  following  characteristics  :  Ab- 
sorption begins  at  X  4730,  rises  to  a  maximum  at  X  4737,  then  falls  to  a 
slight  minimum  at  X  4742,  from  which  it  again  rises  to  a  maximum  at 
X  4755,  falling  off  gradually  to  zero  at  X  4780,  with  indications  of  a  faint 
minimum  near  X  4765.  We  really  have  to  deal  with  a  group  of  three  bands 
then,  their  centers  being  approximately  at  X  4737,  X  4755,  and  X  4772.  With 
dilution  the  bands  at  X  4737  and  X  4772  rapidly  lose  their  identity,  while 
the  band  whose  center  was  at  X  4755  increases  in  intensity  and  somewhat 
asymmetrically,  so  that  in  the  solution  whose  concentration  was  0.99, 
with  a  depth  of  layer  of  9  mm.,  there  remains  but  a  single  band,  its  center 
being  at  X  4760,  and  shading  off  towards  both  sides  a  little  more  than  the 
corresponding  band  in  the  chloride  solution.  With  increasing  dilution 
this  band  also  becomes  more  and  more  like  the  X  4760  chloride  band. 

The  chloride  solution  whose  concentration  was  1.7  normal,  with  a 
layer  3  mm.  deep,  showed  a  deep,  narrow  absorption  band  at  X  5090,  and 
a  wide,  somewhat  hazy  one  with  its  center  at  X  5125.  There  was  a  region 
of  transmission  between  the  two  about  15  A.U.  wide.  A,  Plate  60,  shows 
that  these  bands  do  not  change  materially  with  dilution  to  0.22  normal. 


* 

SALTS    OF    NEODYMIUM,    PRASEODYMIUM,    AND    ERBIUM,  89 

The  corresponding  nitrate  solution  also  shows  a  band  at  X  5090,  but  it  is 
much  wider  and  hazier  than  in  the  chloride  solution,  while  the  X  5125  band 
is,  if  anything,  narrower.  The  two  bands  are  not  clearly  separated  in  the 
first  strip  of  B,  Plate  70.  With  dilution,  however,  the  X  5090  band  narrows 
up  and  becomes  a  little  fainter,  while  the  A  5125  band  widens  a  little  towards 
the  red;  so  that  in  rather  dilute  solutions  the  bands  present  the  same 
appearance  as  they  do  in  the  corresponding  chloride  solutions.  The  region 
X  5200  to  X  5240  shows  practically  continuous  absorption  with  very  hazy 
edges  in  the  first  strip  of  B,  Plate  70  ;  with  dilution  this  changes  rapidly, 
indicating  bands  somewhat  similar  to  those  of  the  chloride  solutions  belong- 
ing to  A,  Plate  60. 

In  A,  Plate  71,  the  band  has  broken  up,  and  instead  of  showing  two 
narrow  intense  bands  at  X  5205  and  X  5222  it  shows  the  following  :  There 
is  a  deep,  narrow  band  at  X  5205,  a  wider  and  very  much  more  intense  one 
at  X  5225,  and  a  rather  narrow,  intense  band  at  X  5235.  With  increasing 
dilution  the  X  5235  band  diminishes  in  intensity,  practically  disappearing 
in  the  most  dilute  solution  used  in  making  B,  Plate  71.  At  the  same  time 
X  5225  decreases  somewhat  in  intensity,  and  rather  more  on  the  red  than 
on  the  violet  side;  so  that  when  the  most  dilute  solution  of  B,  Plate  71,  is 
reached  its  intensity  is  only  slightly  greater  than  that  of  the  X  5205  band 
and  its  center  is  at  about  X  5222.  Here,  then,  we  find  also  the  same  general 
tendency  for  the  spectrum  of  the  nitrate  solutions  to  change  with  dilution  so 
as  to  become  more  and  more  like  that  of  the  chloride  and  bromide  solutions. 

We  might  go  on  and  give  in  detail  the  changes  taking  place  in  the  bands 
located  in  the  yellow,  orange,  and  red,  since  the  changes  here  are  just  as 
well  marked  as  those  we  have  already  described.  But  they  all  point  to 
the  same  thing,  namely,  the  dissimilarity  of  the  spectra  of  concentrated 
solutions,  and  the  gradual  change  of  the  nitrate  spectrum  into  that  of  the 
chloride  or  bromide  with  decreasing  concentration.  That  the  spectra  of 
dilute  solutions  should  become  more  and  more  alike  with  increasing  dilu- 
tion was,  of  course,  to  be  expected  from  the  theory  of  dissociation;  but 
on  the  simple  theory  of  dissociation  no  one  could  have  predicted  that  the 
chloride  and  bromide  should  give  spectra  which  are  practically  identical, 
both  in  concentrated  and  in  dilute  solutions,  while  the  nitrate  should  behave 
so  differently,  especially  as  it  is  well  known  that  the  three  dry  salts  have 
quite  different  absorption  spectra. 

Our  work  on  the  spectrum  of  neodymium  chloride  in  mixtures  of  alco- 
hol and  water  made  it  seem  very  probable  that  the  molecules  as  well  as 
ions  of  the  salt  in  solution  are  solvated,  that  is,  have  combined  with  them 
a  relatively  large  number  of  molecules  of  the  solvent.  On  this  view,  the 
results  with  aqueous  solutions  of  the  chloride,  bromide,  and  nitrate  are 
just  about  what  we  ought  to  expect,  if  we  assume  that  the  absorption 
bands  are  due  to  electrons  which  are  located  in  or  closely  associated  with 
the  neodymium  atom.  Let  us  consider  this  a  little  more  fully,  even  at  the 
risk  of  repeating  certain  things  we  have  said  before. 

Let  the  neodymium  atom  contain  electrons,  which  if  the  atom  is  by 
itself  would  respond  to  light-waves  of  certain  definite  frequencies.  White 
light,  after  having  been  acted  on  by  a  number  of  such  atoms,  would,  when 


90  ABSORPTION   SPECTRA   OF  SOLUTIONS. 

analyzed  by  a  prism  or  grating,  show  a  certain  number  of  absorption  bands 
whose  wave-lengths  could  be  determined.  If,  now,  the  atoms,  instead  of 
being  free,  are  each  united  to  3  chlorine  atoms,  since  these  foreign  atoms 
would  afifect  the  periods  of  the  neodymium  electrons,  we  should  expect  to 
find  the  absorption  spectrum  modified.  If  instead  of  3  chlorine  atoms  we 
had  united  the  neodymium  atom  with  3  bromine  atoms,  we  should  expect  a 
somewhat  different  spectrum  again,  and  so  on  for  the  various  salts  ;  each 
one  would  be  characterized  by  its  own  absorption  spectrum.  If  these  salts 
could  be  dissolved  in  some  medium  which  had  no  action  on  it  except  to 
allow  its  molecules  to  move  about  freely,  we  should  not  expect  any  mate- 
rial change  in  the  spectrum;  while  if  the  solvent  united  with  it,  forming 
solvates,  we  should  expect  the  spectrum  to  be  modified. 

In  a  solvent  like  water,  where  it  is  probable  that  rather  complex  hy- 
drates are  formed,  the  effect  of  the  solvent  might  even  become  the  most 
important  factor  in  determining  the  character  of  the  absorption.  To  take 
a  concrete  case,  suppose  each  molecule  of  a  salt  of  neodymium  in  aqueous 
solution  is  united  with  10  molecules  of  water.  If  the  salt  is  the  chloride 
or  bromide,  each  neodymium  atom  has  only  3  foreign  atoms  to  disturb 
the  periods  of  its  electrons  besides  the  30  atoms  in  the  combined  water; 
while  if  the  salt  is  the  nitrate,  it  would  have  12  foreign  atoms  besides  those 
of  the  water.  Evidently  these  12  atoms  would  have  a  very  much  greater 
effect  than  the  3  in  the  case  of  the  chloride  or  bromide,  if  we  assume  that 
the  general  arrangement  in  space  is  not  very  different  in  the  two  cases. 
We  see,  then,  that  the  fact  that  the  spectrum  of  the  nitrate  in  aqueous 
solutions  of  considerable  concentration  is  different  from  that  of  the  chloride 
or  bromide  is  what  we  should  expect,  and  we  also  see  that  the  very  slight 
change  in  the  spectrum  of  the  bromide  and  chloride  on  dilution,  as  com- 
pared with  the  great  change  in  case  of  the  nitrate,  might  almost  have 
been  predicted. 

The  change  taking  place  with  dilution  is,  of  course,  due  to  dissociation, 
each  neodymium  atom  after  dissociation  being  simply  united  with,  say,  10 
molecules  of  water,  the  anion  of  the  molecule  having  left  it.  The  neodym- 
ium ions  in  dilute  solutions  are,  therefore,  the  same,  no  matter  what  salt 
is  in  solution,  if  we  assume  that  the  presence  of  the  anions  in  the  solution 
does  not  influence  the  hydrating  power  of  the  metallic  ion.  Other  things 
being  equal,  therefore,  we  should  expect  that  salts  whose  molecules  are 
made  up  of  only  a  very  few  atoms  united  with  a  neodymium  atom,  in 
aqueous  solution,  should  show  the  least  change  in  the  spectrum  when  the 
concentration  is  varied;  since  the  removal  of  the  few  atoms  making  up  the 
acid  radical  from  the  hydrated  molecule  would  in  general  have  but  a  slight 
effect  on  the  periods  of  the  absorbing  electrons  in  the  metallic  atom.  Salts 
whose  molecules  consist  of  a  great  many  atoms  united  with  a  neodymium 
atom,  like  the  nitrate,  acetate,  or  sulphate,  when  dissolved  in  water,  ought 
to  show  considerable  change  in  their  spectra  as  a  result  of  dissociation,  since 
the  removal  of  the  great  number  of  atoms  forming  the  acid  radical  would  un- 
doubtedly have  a  marked  influence  on  the  periods  of  the  absorbing  electrons. 

It  is  plain,  therefore,  that  the  theory  outlined  above  furnishes  a  per- 
fectly simple  and  rational  explanation  of  all  the  phenomena  that  have 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM.  91 

thus  far  been  observed  in  the  study  of  the  absorption  spectra  of  neodym- 
ium  salts.  That  it  also  suffices  for  salts  of  the  other  rare  earths  studied 
will  appear  in  what  follows. 

Nbodymium  Nitrate  in  Water — ^Molecules  Constant.     (See  Plate  72  A.) 

The  concentrations  of  the  solutions,  beginning  with  the  one  whose 
spectrum  is  adjacent  to  the  numbered  scale,  were  1.34,  1.08,  0.79,  0.58, 
0.43,  0.34,  and  0.27;  the  corresponding  depths  of  absorbing  layer  being 
3,  4,  6,  9,  13,  18,  and  24  mm. 

As  a  rule  the  bands  all  widen  and  become  somewhat  more  intense  with 
increasing  dilution,  as  might  be  expected  from  the  spectrograms  showing 
the  behavior  of  the  spectrum  when  the  conditions  for  Beer's  law  obtain. 
The  band  at  X  4275,  however,  shows  here  the  same  change  qualitatively 
as  it  did  in  the  series  for  Beer's  law;  that  is,  the  violet  edge  increases 
markedly  in  intensity.  The  red  edge,  however,  remains  of  about  the  same 
intensity  throughout,  indicating  that  it  owes  its  origin  to  the  undissociated 
nitrate  molecules. 

The  X  4330  band,  though  rather  faint,  shows  a  considerable  increase  in 
intensity  with  dilution,  again  indicating  that  it  is  due  to  the  same  absorber 
that  gives  the  violet  edge  of  the  il  4275  band.  .-■:, 

Neodymium  Nitrate  in  Methyl  Alcohol — Beer's  Law.    (See  Plate  73.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  0.80,  0.64,  0.50,  0.40,  0.32,  0.25,  and  0.20;  and  for  B  they  were  0.32, 
0.25,  0.20,  0.16,  0.13,  0.10,  and  0.08;  the  corresponding  depths  of  absorb- 
ing layer  were  6,  7.5,  9.5,  12,  15,  19,  and  24  mm.  in  both  cases. 

On  account  of  the  NO3  band  the  spectrum  terminates  at  X  3250  in  the 
ultra-violet  for  A,  and  at  about  X  3200  for  B. 

The  absorption  near  X  3500  resembles  that  shown  by  aqueous  solutions 
much  more  nearly  than  was  the  case  with  the  chloride.  Only  two  bands 
show,  their  positions  being  X  3465  and  X  3545,  respectively.  The  general 
shading  extends  from  about  X  3450  to  X  3570. 

The  bands  in  the  blue  and  violet  are  not  as  intense  as  the  correspond- 
ing bands  in  the  alcoholic  solution  of  the  chloride.  Their  positions  and 
general  character  are  much  more  nearly  the  same  as  those  shown  by  con- 
centrated solutions  of  the  nitrate  in  water.  There  is  a  band  at  X  4280, 
about  10  A.U.  wide  and  not  specially  intense.  At  X  4430  is  a  wide,  faint 
band,  and  there  is  a  similar  one  at  X  4600.  Three  faint  bands  show  at 
X  4690,  X  4735,  and  X  4825,  resembling  very  much  the  three  corresponding  bands 
in  concentrated  aqueous  solution.    The  intensity  here  is,  however,  much  less. 

In  the  aqueous  solution  we  found  bands  at  X  5205,  X  5225,  and  X  5235, 
of  which  the  first  one  was  evidently  due  to  the  cation,  the  second  one  due 
partly  to  the  cation  and  partly  to  the  molecule,  while  the  one  at  X  5235 
was  apparently  due  only  to  the  nitrate  molecule.  In  the  methyl  alcohol 
solution,  we  find  only  a  weak  shade  in  the  region  X  5200,  while  at  X  5225 
and  X  5240  there  are  two  rather  narrow,  intense  bands.  There  is  consider- 
able shading  to  both  sides  of  these  bands. 


92  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

In  the  yellow,  A  shows  absorption  from  X  5700  to  X  5870,  shading  off 
towards  the  red,  with  a  band  at  X  5965.  B  shows  a  band  at  X  5720,  which 
is  perhaps  double;  deep  absorption  from  X  5755  to  X  5845,  with  faint  bands 
at  X  5760  and  X  5835,  and  a  very  intense  band  at  X  5790. 

The  spectrum  ends  near  X  7320  in  a  band,  which  does  not  seem  espe- 
cially intense. 

Neodymium  Nitrate  in  Ethyl  Alcohol — ^Beer's  Law.    (See  Plate  74.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  0.32,  0.26,  0.20,  0.16,  0.13,  0.10,  and  0.08,  and  for  B  they  were  in  the 
same  order,  0.16,  0.13,  0.10,  0.08,  0.06,  0.05,  and  0.04;  the  corresponding 
depths  of  absorbing  layer  were  6,  7.5,  9.5,  12,  15,  19,  and  24  mm. 

The  solutions  used  in  making  A  of  this  plate  had  the  same  concentra- 
tions as  those  used  in  making  B  of  Plate  73,  and  as  the  depths  of  absorbing 
layer  were  also  the  same,  the  two  plates  are  directly  comparable. 

The  two  spectra  are  very  similar,  but  nevertheless  there  are  some  well- 
marked  differences.  The  bands  at  X  5225  and  X  5240,  which  were  quite 
sharp  and  intense  in  the  methyl  alcohol  solution,  here  show  simply  as  one 
hazy  band  of  moderate  intensity,  its  middle  being  near  X  5235.  Even  in 
B,  where  the  concentration  is  much  less,  this  band  does  not  break  up  into 
two,  but  simply  diminishes  in  intensity  without  change  of  character. 

The  yellow  group  shows  a  wide,  faint  band  at  X  5730,  a  moderately 
intense  band  at  about  X  5790,  much  less  intense  and  sharp  than  in  methyl 
alcohol.  There  is  a  pair  of  poorly  defined  bands  at  X  5825  and  X  5845, 
apparently  corresponding  to  the  band  at  X  5835,  observed  in  the  solutions 
in  methyl  alcohol. 

The  spectrum  ends  at  X  7315  in  a  band  which  is  not  very  intense  or 
sharp.  In  general,  the  absorption  in  the  two  alcohols  is  about  the  same, 
the  tendency  being  for  all  absorption  bands  to  be  narrower  in  the  methyl 
alcohol  than  in  the  ethyl  alcohol  solutions. 

Neodtmium  Nitrate  in  Acetone — Beer's  Law.     (See  Plate  75.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  0.60,  0.48,  0.37,  0.30,  0.24,  0.19,  and  0.15.  For  B  they  were 
0.19,  0.15,  0.12,  0.095,  0.075,  0.060,  and  0.047;  the  depths  of  absorbing 
layer  were  in  both  cases  6,  7.5,  9.5,  12,  15,  19,  and  24  mm. 

The  nitrate  was  found  to  be  much  more  soluble  in  acetone  than  in  ethyl 
alcohol,  being  in  this  respect  quite  different  from  the  chloride,  which,  when 
anhydrous,  dissolves  quite  readily  in  ethyl  alcohol,  but  scarcely  at  all  in 
acetone. 

The  spectrum  in  the  ultra-violet  ends  at  about  X  3300,  as  is  usual  in 
acetone  solutions.  The  bands  in  the  ultra-violet  absorption  near  X  3500 
have  the  positions  X  3475  and  X  3555,  are  both  rather  faint,  and  have  a 
width  of  about  15  or  20  A.U.  They  are  hence  both  wider  and  fainter  than 
they  were  in  the  methyl  alcohol  solution.  Their  position  is  apparently 
about  10  A.U.  nearer  the  red  end  of  the  spectrum  than  was  the  case  in  the 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,    AND    ERBIUM.  93 

alcoholic  solutions,  but  the  greater  part  of  this  is  perhaps  due  to  the  broad- 
ening, which  is  somewhat  unsymmetrical. 

The  band  at  X  4280  is  about  15  A.U.  wide  and  not  very  intense.  The 
other  bands  in  the  violet,  blue,  and  blue-green  are  so  faint  as  to  make 
measurements  impossible.  Apparently  they  agree  pretty  well  in  general 
appearance  and  position  with  the  corresponding  bands  in  methyl  alcohol. 
However,  much  deeper  layers  of  the  solution  than  could  possibly  be  used 
with  the  apparatus  employed  in  the  present  investigation  would  be  needed 
in  order  to  study  these  bands  at  all  carefully. 

At  \  5110  there  is  a  fairly  intense,  but  wide  and  hazy  band.  X  5215  is 
another  similar  in  appearance  to  the  one  at  X  5110,  though  not  quite  as 
hazy.  It  is  not  entirely  separated  from  the  much  more  intense  band  at 
X  5255.  The  latter  corresponds  to  the  doublet  X  5225  and  X  5240  in  methyl 
alcohol,  and  the  hazy  band  at  about  X  5235  in  ethyl  alcohol.  Its  position 
is  therefore  somewhat  nearer  the  red  end  of  the  spectrum. 

In  the  yellow  A  shows  absorption  from  X  5690  to  X  5900.  At  X  6020  is  a 
moderately  intense  but  rather  wide  band,  which  has  a  fainter  and  narrower 
companion  at  X  6040. 

There  is  a  set  of  bands  in  the  region  X  6100  to  X  6300,  which  seems 
to  increase  somewhat  in  intensity  towards  the  red;  but  the  absorption  is 
too  faint  to  allow  the  individual  bands  to  be  picked  out.  There  is  a  moder- 
ately intense  but  hazy  band  at  X  6760.  The  spectrum  ends  near  X  7300. 
B  shows  the  yellow  group  broken  up  into  two  moderately  intense  but 
rather  wide  bands  at  X  5725  and  X  5775,  and  a  much  wider  and  stronger 
band,  with  its  center  at  X  5840,  which  is  strongly  shaded  to  both  sides. 
Indications  are  that  this  band  is  at  least  double,  the  more  intense  compo- 
nent being  towards  the  violet.  The  spectrum  shown  in  B  ends  at  X  7315, 
and  there  is  a  slight  indication  that  between  this  point  and  X  7400  there  is 
a  group  of  three  or  more  bands. 

In  general,  it  appears  that  as  the  molecular  weight  of  the  solvent  is 
increased  the  absorption  bands  become  wider  and  wider.  In  aqueous 
solutions  there  are  a  number  of  bands  having  a  width  of  only  a  few  Ang- 
strom units,  while  in  methyl  alcohol  few  bands  are  narrower  than  from 
8  to  12  units.  In  ethyl  alcohol  no  band  is  narrower  than  10  to  15  units, 
and  in  the  acetone  their  width  is  still  greater. 

Neodyaoum  Nitrate  in  Mixtuees  of  Methyl  Alcohol  and  Wateb. 
(See  Plate  76  A.) 

The  concentration  of  the  neodymium  nitrate  was  constant  throughout 
and  equal  to  0.5  normal.  The  percentages  of  water  in  the  solutions,  begin- 
ning with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale,  were 
0,  16.6,  33.3,  50,  66.6,  83.3,  and  100  per  cent.  The  common  depth  of 
absorbing  layer  was  0.5  cm. 

The  changes  here  are  similar  to  those  discussed  in  considering  Plate  65; 
that  is,  the  change  from  the  bands  characteristic  of  the  aqueous  solution  to 
those  belonging  to  the  alcoholic  solution  takes  place  in  passing  from  the 
solution  containing  16.6  per  cent  of  water  to  the  one  containing  no  water. 
The  spectrogram,  however,  shows  that  the  spectrum  changes  consider- 


94  ABSORPTION  SPECTRA  OF  SOLUTIONS. 

ably  from  solution  to  solution,  even  when  the  percentage  of  water  is  much 
greater;  for  example,  changes  may  be  noticed  in  passing  from  the  solution 
containing  100  per  cent  water  to  the  one  containing  50  per  cent.  This  is 
undoubtedly  due  to  the  change  in  the  dissociation  of  the  dissolved  salt, 
which,  in  the  case  of  the  nitrate,  modifies  the  spectrum;  while  in  the 
case  of  the  chloride  no  such  change  was  noted,  except  at  the  very  greatest 
concentrations.  The  spectrogram,  therefore,  shows  a  superposition  of  the 
two  effects,  and  if  this  is  borne  in  mind  everything  about  it  is  perfectly 
clear  without  further  discussion. 

Neodymidm  Nitrate  in  Mixtures  op  Acetone  and  Water.    (See  Plate  67  B.) 

The  concentration  of  the  neodymium  nitrate  was  constant  throughout 
and  equal  to  0.6  normal.  The  percentages  of  water  in  the  solutions,  begin- 
ning with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale,  were 
0,  2.6,  5.3,  8,  10.6,  13.3,  and  16.  The  common  depth  of  absorbing  layer  for 
all  the  solutions  was  0.5  cm. 

In  this,  as  well  as  in  the  case  treated  under  Plate  76  A,  we  have  to  do 
with  a  superposition  of  two  effects.  First,  the  change  produced  in  the 
water  solution  resulting  from  decreased  dissociation  with  the  addition  of 
the  non-aqueous  solvent;  and  secondly,  the  change  in  the  structure  of 
the  bands  which  takes  place  when  the  amount  of  water  has  been  decreased 
so  far  that  the  molecules  of  the  dissolved  substance  are  no  longer  able  to 
be  surrounded  by  the  usual  number  of  water-molecules,  but  become  sur- 
rounded by  molecules  of  the  non-aqueous  solvent — ^in  the  present  case 
acetone.  In  the  solution  whose  spectrum  is  nearest  the  narrow,  comparison 
spark  spectrum,  the  percentage  of  water  being  only  16  per  cent,  the  dis- 
sociation is  already  rather  slight,  so  that  the  spectrum  is  approximately 
that  which  we  would  observe  in  a  very  concentrated  aqueous  solution  of 
the  salt  in  a  layer  only  about  a  millimeter  in  depth.  With  decrease  in  the 
amount  of  water  the  change  is  easiest  to  follow  in  the  more  intense  of  the 
bands  in  the  green,  this  being  the  one  which  differs  most  in  the  acetone 
and  concentrated  aqueous  solutions.  It  will  be  noticed  that  the  most 
marked  change  in  this  band  takes  place  in  passing  from  the  fifth  to  the 
third  strips,  counting  from  the  scale;  that  is,  when  the  water  content  of 
the  solvent  changes  from  10.6  to  5.3  per  cent.  This  agrees  substantially 
with  what  we  found  to  hold  in  the  case  of  solutions  of  the  chloride  in  mix- 
tures of  water  and  the  alcohols. 

Praseodymium  Chloride  in  Water — ^Beer's  Law.    (See  Plate  77.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  2.56,  1.92,  1.28,  0.85,  0.60,  0.42,  and  0.32.  For  B  the  concen- 
trations were  0.85,  0.63,  0.42,  0.28,  0.20,  0.14,  and  0.11,  the  depths  of 
absorbing  layer  being  respectively  3,  4,  6,  9,  13,  18,  and  24  mm. 

The  solutions  of  praseodymium  chloride  are  all  green  or  yellowish-green, 
only  the  intensity  of  the  color  changing  with  change  in  the  concentration. 

For  these  solutions  Beer's  law  holds  very  exactly,  excepting  for  the 
extreme  ultra-violet  absorption  in  A,  and  the  yellow  bands  in  the  two  or 
three  most  concentrated  solutions  of  A. 


0 

SALTS    OF    NEODYMIUM,    PRASEODYMIUM,    AND    ERBIUM.  95 

The  limits  of  transmission  in  the  ultra-violet,  for  the  most  concentrated 
and  most  dilute  solutions  of  A,  are,  respectively,  X  2720  and  X  2650.  The 
edge  is  fairly  sharp,  indicating  the  presence  of  a  rather  intense  band. 
This  is  also  indicated  by  B,  where  the  spectrum  ends  abruptly  at  X  2630, 
the  limit  being  the  same  for  all  of  the  solutions. 

The  absorption  bands  shown  in  A  are  as  follows:  X  4380  to  X  4480, 
strong  band  with  red  edge  somewhat  shaded;  X  4640  to  X  4710,  sharp  on 
red  side,  quite  diffuse  towards  the  violet;  X  4800  to  X  4830,  sharply  defined 
on  both  sides;  X  5860  to  X  5950,  both  edges  diffuse;  X  5985,  fairly  narrow 
band  with  diffuse  edges.  The  region  between  this  band  and  the  principal 
yellow  one  shows  very  strong  absorption. 

B  shows  the  following:  X  4410  to  X  4465,  both  edges  a  little  diffuse; 
X  4685,  fairly  narrow  band,  still  more  diffuse  towards  the  violet,  although 
somewhat  shaded  also  towards  the  red;  X  4815,  narrow  band,  with  edges 
slightly  shaded;  X  5900,  wide  hazy  band;  absorption  not  complete,  even 
at  its  middle;  X  5985,  rather  faint,  hazy  band. 

The  greenish  tinge  of  the  solutions  would  suggest  that  there  is  con- 
siderable general  absorption  in  the  red,  because  the  absorption  in  the 
yellow  is  not  sufficient  to  impart  any  marked  color  to  the  solution,  and 
the  bands  in  the  violet  and  blue  could  only  give  it  a  yellow  tint.  The  nega- 
tive for  A  does,  in  fact,  show  pretty  strong  general  absorption  from  ^7100 
to  the  end  of  the  red,  but  no  doubt  a  spectrophotometric  study  of  the 
solutions  would  show  general  absorption  much  farther  down  into  the  red. 
The  negative  for  B  shows  no  sign  of  this  absorption,  for  very  obvious  reasons. 

Pkaseodtmtom  Chloride  in  Mixtures  op  the  Alcohols  and  Water. 
(See  Plate  78.) 

The  concentration  of  the  praseodymium  chloride  was  constant  through- 
out and  equal  to  0.5  normal.  The  percentages  of  water  in  the  solutions, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  0,  2.3,  5.6,  8,  10.6,  13.3,  and  16.  The  depth  of  absorbing  layer  was 
1.0  cm.  Methyl  alcohol  was  the  chief  solvent  in  the  solutions  pertaining 
to  A,  while  ethyl  alcohol  was  employed  in  the  solutions  used  in  making 
the  negative  for  B.  The  two  spectrograms  are  identical,  except  for  a  little 
greater  general  absorption  in  the  ultra-violet  with  the  ethyl  alcohol.  The 
most  striking  feature  of  the  spectrograms  is  the  appearance  of  the  intense 
absorption  band  near  X  3000  as  the  percentage  of  water  is  gradually 
decreased.  Only  a  faint  trace  of  this  band  is  visible  with  16  per  cent  of 
water  in  the  solution,  and  the  band  is  comparatively  weak  even  with  only 
8  per  cent  of  water.  From  this  point  it  increases  very  rapidly  in  width 
and  intensity  with  decrease  in  the  amount  of  water,  until  in  the  pure 
alcohol  solutions  its  limits  (transmission)  are  X  2970  and  i^  3230,  being  by 
far  the  most  intense  band  in  the  whole  spectrum. 

The  bands  in  the  violet  and  blue  apparently  shift  somewhat  towards 
the  red,  this  being,  however,  due  to  the  fact  that  the  alcohol  bands  are  a 
little  nearer  the  red  end  of  the  spectrum,  and  that  when  the  percentage 
of  water  changes  from  16  to  0,  the  two  sets  of  bands  coexist,  but  are  far 
from  being  separated.    The  change  is  exactly  the  same  in  character  as  the 


96  ABSORPTION   SPECTRA   OF   SOLUTIONS. 

one  described  in  detail  in  discussing  the  X  4760  band  for  neodymium 
chloride  in  mixtures  of  alcohol  and  water. 

The  positions  of  the  bands  in  the  solution  containing  16  per  cent  of 
water  are  as  follows  :  X  4390  to  X  4470,  X  4660  to  X  4700,  X  4800  to  X  4825. 
In  the  solution  in  pure  alcohol  they  are  X  4410  to  X  4480,  X  4690  to  X  4715, 
X  4810  to  X  4840.  Hence,  it  appears  that  the  two  most  refrangible  bands 
have  a  slightly  greater  width  in  the  water  solution,  while  the  X  4815  band 
is  more  intense  in  the  alcoholic  solutions. 

The  bands  in  the  yellow  show  very  well  indeed  the  fact  that  here  as 
in  the  spectrum  of  neodymium  chloride  we  have  the  coexistence  of  two 
sets  of  bands  when  the  water  content  of  a  0.5  normal  solution  is  in  the 
neighborhood  of  8  per  cent.  The  band  in  the  yellow  has  already  been 
described  under  Beer's  law,  but  as  the  concentration  and  depth  of  layer 
are  different  here,  the  following  will  serve  to  indicate  what  the  spectrum 
of  the  16  per  cent  water  solution  shows.  Absorption  begins  at  X  5850  and 
rises  to  a  maximum  at  about  X  5900,  then  decreases  to  a  minimum  at  X 
5950,  from  which  it  again  rises  to  a  maximum  at  about  X  5980,  falling  off 
to  zero  at  X  6000.  The  solution  in  pure  alcohol  shows  the  following  :  Weak 
absorption  begins  at  X  5800  and  continues  without  material  change  up  to 
X  5880,  where  it  falls  almost  to  nothing.  At  X  5900  it  begins  to  increase 
and  reaches  a  strong  maximum  at  X  5955,  falling  off  gradually  to  zero  at 
X  6000.  The  intermediate  solutions  show  the  gradual  disappearance  of 
the  bands  characteristic  of  the  aqueous  solution,  and  the  increase  in  inten- 
sity of  those  belonging  to  the  alcoholic  solution,  as  the  percentage  of  water 
is  gradually  decreased.  The  maximum  change  takes  place  from  the  fifth 
to  the  third  strips,  counting  from  the  numbered  scale,  indicating  here,  as 
with  neodymium  chloride,  that  the  two  sets  have  about  half  their  normal 
intensity  when  the  water  content  of  the  solution  is  about  8  per  cent,  or 
when  the  solution  contains  about  10  molecules  of  water  per  molecule  of 
the  dissolved  substance. 

Pbaseodtmium  Nitrate  in  Water — Beer's  Law.     (See  Plate  79.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for 
A,  beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered 
scale,  were  3.2,  2.4,  1.6,  1.1,  0.75,  0.53,  and  0.41.  For  B  the  concentrations 
were  1.1,  0.80,  0.55,  0.33,  0.26,  0.18,  and  0.14;  the  depths  of  absorbing 
layer  in  both  cases  were  3,  4,  6,  9,  13,  18,  and  24  mm. 

There  is  a  great  deal  of  absorption  in  the  ultra-violet,  the  spectrum  of 
the  most  concentrated  solution  ending  at  i^  4000,  while  that  of  the  four 
most  dilute  solutions  of  set  A  ends  at  about  X  3650.  The  spectra  shown 
in  B  all  end  at  X  3570.  This  absorption  is  not  to  be  ascribed  to  the  NO, 
radical,  as  its  band  lies  beyond  X  3300  in  all  the  solutions  thus  far  studied. 

The  absorption  bands  do  not  differ  materially  from  those  of  the  chlo- 
ride, except  that  they  are  a  trifle  more  intense,  due,  no  doubt,  to  the  slightly 
greater  concentration  of  the  nitrate  solutions.  Also,  the  violet  and  blue 
bands  show  a  slight  deviation  from  Beer's  law  in  the  two  or  three  most 
concentrated  solutions  of  A. 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM.  97 

Erbium  Chloride  in  Water — ^Beer's  Law.     (See  Plate  80.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  1.4,  1.18,  0.98,  0.80,  0.66,  0.56,  and  0.47.  The  concentrations  for  B 
were  0.80,  0.67,  0.56,  0.46,  0.38,  0.32,  and  0.27,  the  corresponding  depths 
of  absorbing  layer  being  8,  9.5,  11.5,  14,  17,  and  24  mm. 

The  concentrations  here  given  were  obtained  by  assuming  the  atomic 
weight  of  the  metallic  atom  as  166,  that  is,  assuming  that  the  solution  was 
one  of  pure  erbium  chloride.  As  the  salt  contained  very  large  quantities 
of  yttrium  and  other  related  elements,  the  figures  given  for  the  concentra- 
tion can  have  no  meaning  in  the  absolute  sense.  They  merely  indicate  the 
relative  amounts  of  erbium  chloride  in  the  different  solutions  employed. 

For  these  solutions  of  the  chloride  Beer's  law  holds  pretty  accurately, 
excepting  for  the  absorption  in  the  extreme  ultra-violet,  where  the  limits 
of  transmission  for  the  most  concentrated  and  most  dilute  solutions  of  A 
are  X  2870  and  X  2760.  For  B  the  corresponding  figures  are  X  2760  and 
X  2650. 

The  positions  of  the  chief  bands  are  as  follows:  X  3240,  X  3500,  X  3635, 
and  X  3785  in  the  region  covered  by  the  spark  spectrum;  X  4150,  X  4210, 
X  4415,  X  4495  moderately  strong,  X  4515  fairly  intense,  X  4670  very  faint, 
X  4845,  X  4865  intense,  >l  4905,  X  5185  faint,  X  5205  fairly  intense,  X  5230 
intense,  X  5365,  X  5415,  X  5435,  X  5490  faint,  X  6410  faint,  >l  6490  faint,  X  6535 
fairly  intense,  and  X  6680  rather  faint. 

Erbium  Nitrate  in  Water — Beer's  Law.     (See  Plate  81.) 

The  concentrations  of  the  solutions  used  in  making  the  negative  for  A, 
beginning  with  the  one  whose  spectrum  is  adjacent  to  the  numbered  scale, 
were  1.4,  1.05,  0.88,  0.70,  0.56,  0.44,  and  0.35.  For  B  the  concentrations 
were  0.70,  0.52,  0.44,  0.35,  0.28,  0.22,  and  0.17,  the  depths  of  absorbing 
layer  being  in  both  cases  6,  7.5,  9.5,  12,  15,  19,  and  24  mm. 

What  was  said  about  the  significance  of  the  figures  given  for  the  con- 
centrations under  erbium  chloride,  applies  equally  well  in  this  case,  since 
the  same  material  was  used.  Here  the  ultra-violet  is  limited  by  the  NOg 
band  at  about  X  3300  as  usual. 

The  more  concentrated  solutions  give  a  spectrum  which  is  somewhat 
different  from  that  produced  by  the  chloride  solutions.  The  bands  are  as 
a  rule  wider  and  hazier,  and  their  intensity  maxima  sometimes  fall  in  slightly 
different  positions.  With  dilution  the  character  of  the  bands  changes  con- 
siderably, becoming  more  and  more  like  the  bands  given  by  the  chloride 
solution.  Here,  again,  then,  we  find  a  state  of  affairs  very  like  the  one  we 
discussed  at  some  length  under  neodymium  nitrate — Beer's  law. 

Judging  from  the  negatives  made  with  the  solutions  of  erbium  salts, 
it  appears  that  the  absorption  spectrum  of  erbium  would  make  fully  as 
interesting  a  study  as  that  of  neodymium,  and  it  is  to  be  hoped  that  in 
the  continuation  of  this  work  some  preparation  richer  in  erbium  than  the 
one  we  employed  will  be  available. 


98  ABSOBPTION    SPECTRA   OF    SOLUTIONS. 

It  may  be  said  in  general  that  the  absorption  spectra  of  the  different 
salts  of  the  same  metal  resemble  each  other  very  closely;  and  it  is  only 
when  careful  attention  is  paid  to  the  structure  of  each  individual  band, 
or  group  of  bands,  that  the  differences  are  brought  out  clearly.  In  the 
process  of  printing  and  reproducing  the  spectrograms  illustrating  this 
chapter,  a  great  deal  of  the  finer  detail  has  been  lost,  and  as  it  is  just  this 
detail  which  shows  the  differences  alluded  to  above,  it  is  clear  that  in 
many  cases  the  reproductions  fail  entirely  to  show  the  important  points. 
In  cases  of  special  importance  a  rather  full  description  of  the  appearance 
as  seen  on  the  negatives  has  been  given  in  the  text.  A  full  description 
of  this  nature  covering  all  the  spectrograms  of  the  present  chapter  would 
require  an  amount  of  time  and  space  that  would  be  quite  prohibitive, 
and  unnecessary. 

It  is  hoped,  however,  that  the  plates,  together  with  the  description  given 
in  the  text,  will  make  clear  the  points  which  we  have  tried  to  emphasize 
most  strongly,  viz: 

1.  That  the  absorption  spectra  of  different  salts  of  the  same  metal  in 
the  same  solvent  are  different  if  the  concentration  is  great,  or,  more  gener- 
ally, if  the  dissociation  is  only  slight;  and  that  as  the  dissociation  becomes 
more  and  more  complete,  they  become  more  and  more  alike. 

2.  That  the  absorption  spectra  of  the  same  salt  in  different  solvents 
are  in  general  different. 

3.  That  with  change  in  dissociation  of  the  salt  in  any  one  solvent,  the 
change  in  the  absorption  spectrum  of  salts  having  anions  containing  only 
a  few  atoms,  such  as  the  chloride  and  bromide,  is  very  slight;  but  that 
as  the  complexity  of  the  anion  increases,  the  change  becomes  more  and 
more  pronounced. 

4.  That  when  a  salt  is  dissolved  in  mixtures  of  two  solvents,  the  rela- 
tive percentages  of  which  are  varied,  there  is  not  a  gradual  change  of  one 
spectrum  into  the  other;  but  the  spectrum  given  by  the  mixture  is  a  super- 
position of  the  two  spectra,  the  two  sets  of  bands  existing  together.  If 
the  salt  is  one  whose  spectrum  changes  considerably  with  its  state  of 
dissociation,  we  have  in  addition  to  the  above  phenomena  the  changes  due 
to  the  varying  dissociation  of  the  dissolved  salt  produced  by  the  varying 
composition  of  the  mixture. 

The  explanation  of  these  points  on  the  working  hypothesis  which  has 
guided  the  present  work,  has  already  been  given  in  the  discussion  of  Plates 
70  and  71. 

In  the  introduction  to  the  present  chapter  the  work  of  Helen  Schaeffer 
was  referred  to.  It  will  be  recalled  that  she  studied  the  spectrum  of  the 
nitrate  of  neodymium  in  various  solvents,  and  also  in  mixtures  of  two 
solvents;  the  case  to  which  she  calls  special  attention  being  mixtures  in 
various  proportions  of  water  and  acetone.  She  did  not  come  to  the  con- 
clusion which  we  have  reached,  that  in  such  mixtures  we  have  two  distinct 
sets  of  absorption  bands,  since  she  considers  the  bands  as  shifting  gradu- 
ally, some  in  the  direction  demanded  by  Kundt's  law,  and  some  in  the 
opposite  direction.  There  are  two  reasons  why  she  did  not  come  to  the 
same  conclusion  that  we  have  reached.    In  the  first  place  she  worked  with 


SALTS    OF    NEODYMIUM,    PRASEODYMIUM,   AND    ERBIUM.  99 

the  nitrate,  which  we  have  found  shows  a  considerable  variation  in  its 
spectrum  with  change  in  dissociation,  and  her  solvents  being  water  and 
acetone,  the  change  in  the  dissociation  would  be  very  considerable.  For 
this  reason,  she  found  a  continuous  change  in  the  spectrum  as  more  and 
more  acetone  was  added,  which  was  just  what  she  expected.  Had  she 
worked  with  the  chloride  or  bromide  she  would  have  found  practically  no 
change  until  the  proportion  of  the  non-aqueous  solvent  in  the  mixture 
had  become  very  great,  and  in  this  event  her  conclusions  would  have 
been  quite  different. 

In  the  second  case  her  salts  were  not  dehydrated  (if  they  were  she 
makes  no  mention  of  the  fact),  and  hence  even  in  the  solution  in  pure 
acetone  she  probably  had  from  6  to  10  molecules  of  water  per  molecule  of 
the  dissolved  salt,  which  we  have  found  would  give  the  spectrum  character- 
istic of  the  non-aqueous  solvent  with  only  about  half  its  normal  intensity. 
It  is  not  very  surprising,  therefore,  that  she  failed  to  discover  the  coexist- 
ence of  the  two  sets  of  bands,  which  would  have  given  a  perfectly  simple 
explanation  of  all  the  phenomena  that  she  observed. 


CHAPTER  VIII. 

SUMMARY  AND  CONCLUSIONS. 

It  is  evident  from  the  spectra  of  the  solutions  studied  in  the  present 
investigation  that  deviation  from  Beer's  law  is  the  rule  rather  than  the 
exception.  Of  the  great  number  of  sets  of  solutions  studied,  only  a  very- 
limited  number  appear  to  confirm  Beer's  law,  and  it  is  possible  that  with 
the  more  exact  spectrophotometric  measurements  this  number  would  be 
reduced  still  further.  This  is  exactly  what  we  should  expect,  since  actual 
solutions  always  contain  more  than  one  kind  of  "absorber,"  and  the  rela- 
tive concentrations  of  these  "absorbers"  are  continually  changing  with 
change  in  concentration  of  the  solution.  Beer's  law  could  only  hold,  as 
explained  in  the  introductory  chapter,  in  cases  where  the  relative  con- 
centrations of  the  different  kinds  of  absorbers  do  not  change  with  dilution, 
or  in  the  event  that  the  absorption  of  all  the  different  kinds  of  absorbers 
is  identical.  The  first  one  of  these  conditions  is  perhaps  never  fulfilled, 
while  the  second  one  is  undoubtedly  approached  more  or  less  closely  in 
certain  cases,  such  as  in  aqueous  solutions  of  neodymium  chloride  or  bro- 
mide or  of  praseodymium  chloride.  The  rule  is,  however,  that  the  different 
absorbers  have  different  absorbing  powers,  and  the  problem  is,  therefore, 
to  decide  which  absorbers  are  responsible  for  the  bands  observed  in  the 
various  spectra. 

According  to  the  theory  of  Ostwald,  which  is  simply  Arrhenius's  dis- 
sociation theory  applied  to  the  absorption  spectra  of  solutions,  we  have 
but  two  or  three  kinds  of  absorbers,  namely,  the  molecules  of  the  dissolved 
salt  and  one  or  both  the  ions  formed  from  it.  In  the  case  of  all  the  salts 
studied  in  the  present  work,  excepting  the  nitrates,  the  anion  has  been 
colorless;  so  all  the  absorption,  according  to  Ostwald's  theory,  should  be 
due  to  two  kinds  of  absorbers,  the  molecule  and  the  cation.  That  this 
theory  fails  entirely  to  account  for  the  deviation  from  Beer's  law  observed 
in  the  ultra-violet  absorption  of  copper  salts,  the  red  bands  of  cobalt  salts, 
the  ultra-violet  band  of  cobalt  chloride,  and  the  absorption  of  iron  chloride, 
has  already  been  pointed  out;  since  all  of  these  bands  narrow  with  dilution, 
even  when  the  number  of  molecules  in  the  path  of  the  beam  of  light  is  kept 
constant.  Whether  this  theory  is  able  to  account  for  the  behavior  of  those 
bands  which  narrow  with  dilution  when  the  conditions  for  Beer's  law 
obtain,  but  which  widen  when  molecules  are  kept  constant,  can  only  be 
decided  by  spectrophotometric  measurements. 

The  work  of  Miiller  on  salts  of  nickel  and  copper  shows  that  the  behavior 
of  the  red  absorption  band  of  these  substances  can  not  be  accounted  for 
on  Ostwald's  theory,  and  this  makes  it  at  least  very  probable  that  the  same 
will  be  found  for  salts  of  other  metals.  Ostwald's  theory  may,  therefore, 
be  dismissed,  not  because  it  is  erroneous,  but  because  it  is  incomplete. 
It  leaves  out  of  account  certain  changes  taking  place  in  solutions,  which 
produce  other  "absorbers"  than  those  which  it  considers. 
100 


SUMMARY  AND    CONCLUSIONS.  101 

The  other  theories  which  aim  to  account  for  the  deviations  are  of  two 
kinds,  viz: 

(1)  Those  that  assume  that  the  increased  absorption  in  concentrated 
solutions  is  due  to  the  formation  of  aggregates  of  the  molecules  of  the 
dissolved  substance,  or  of  the  molecules  and  the  ions  into  which  they 
break  down  on  dissociation. 

(2)  Those  that  assume  that  the  deviation  is  due  to  the  formation  of 
solvates,  that  is,  combinations  of  the  parts  of  the  dissolved  substance  with 
the  molecules  of  the  solvent. 

It  has  been  shown  by  Hartley  and  other  workers  who  have  studied  the 
change  in  the  absorption  with  change  in  temperature,  that  the  bands  which 
widen  with  increase  in  concentration  (conditions  for  Beer's  law  assumed 
to  obtain)  also  widen  with  rise  in  temperature;  that  is,  a  rise  in  tempera- 
ture produces  very  much  the  same  effect  as  increase  in  concentration.  This 
seems  to  us  pretty  conclusive  evidence  against  the  theories  that  are  based 
on  the  formation  of  aggregates,  for  it  is  well  known  that  the  change  in  the 
aggregates  produced  by  rise  in  temperature  is  not  the  same  as  that  produced 
by  increase  in  concentration,  but  exactly  the  opposite. 

The  theories  which  assume  the  formation  of  solvates  are  not  open  to 
this  objection,  because  it  is  well  known  that  the  change  in  the  solvates 
produced  by  rise  in  temperature  is  in  general  the  same  as  that  produced 
by  increase  in  concentration.  As  a  solution  becomes  more  concentrated 
the  solvates  become  simpler  and  simpler,  that  is,  fewer  molecules  of  the 
solvent  are  combined  with  each  part  of  the  dissolved  substance.  Rise  in 
temperature  also  breaks  down  complex  solvates  into  simpler  ones.  Of 
course,  it  does  not  follow  that  the  solvates  of  a  solution  of  concentration 
Cj  at  temperature  t^  are  exactly  the  same  as  those  in  a  solution  of  concen- 
tration Cj  at  a  temperature  ^2;  since  under  the  changed  conditions  it  may 
happen  that  the  particular  solvates  which  were  most  stable  when  the 
conditions  were  c^  and  ti  may  be  less  stable  than  solvates  of  nearly  the 
same  composition  at  C2,  t^. 

For  this  reason,  and  also  because  our  work  on  neodymium  and  praseo- 
dymium salts  in  mixed  solvates  seems  almost  conclusive  evidence  in  favor  of 
the  existence  of  solvates,  we  have  used  the  solvate  theory  as  a  working 
hypothesis  throughout  this  investigation.  That  it  is  not  far  from  being 
correct  is  shown  by  the  fact  that  all  the  phenomena  observed  in  the  great 
number  of  solutions  studied  are  accounted  for  without  anjrthing  but  the 
simplest  assumptions  in  regard  to  the  behavior  of  the  solvates  in  question. 

We  shall  now  summarize  briefly  the  main  points  brought  out  in  the 
present  work. 

Solutions  of  cobalt  salts  have,  in  general,  three  regions  of  absorption 
in  that  part  of  the  spectrum  which  can  be  photographed  without  resorting 
to  other  means  than  the  commercial  dry  plate.  One  is  in  the  extreme 
ultra-violet,  and  we  concluded  that  it  is  due  to  the  molecules  of  the  dis- 
solved substance.  Their  absorption  is  influenced  to  some  slight  extent  by 
solvation,  but  differently  for  the  different  salts.  That  no  part  of  this 
absorption  is  due  to  the  cobalt  ions  is  shown  by  the  fact  that  solutions  of 
cobalt  sulphate  are  perfectly  transparent  beyond  X  2200,  although  they  are 
dissociated  to  a  very  considerable  extent. 


102  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

Another  region  of  absorption  is  in  the  green,  near  X  5200,  for  most 
solutions.  This  band,  which  is  the  most  characteristic  one  of  cobalt  solu- 
tions, was  ascribed  by  Ostwald  to  the  cobalt  ion.  That  the  molecules  also 
absorb  in  this  region,  and  in  fact  have  a  greater  absorbing  power  than  the 
ion,  has  been  abundantly  shown  in  Chapter  II.  Whether  the  simple  theory 
of  dissociation  is  able  to  account  for  the  observed  deviations  from  Beer's 
law  for  this  band  is  not  known,  but  is  improbable.  The  question  is  now 
being  investigated  in  this  laboratory,  and  a  definite  answer  will  probably 
be  given  in  the  near  future. 

The  absorption  band  at  X  3300  in  the  spectrum  of  the  aqueous  solution 
of  cobalt  chloride,  since  it  disappears  with  dilution  even  when  molecules 
are  kept  constant,  can  not  be  due  to  the  cobalt  chloride  molecules;  but  we 
found  good  reasons  for  thinking  that  it  is  due  to  some  hydrate  of  these 
molecules  which  is  formed  in  solutions  of  moderate  concentration  even  at 
ordinary  temperature.  The  two  bands  in  the  same  region,  which  appear 
in  the  alcoholic  solutions  of  the  same  salt,  behave  so  much  like  the  band 
in  the  aqueous  solution  that  they  are  undoubtedly  due  to  some  relatively 
simple  alcoholate. 

The  bands  in  the  red  region  of  the  spectrum  of  solutions  of  cobalt  salts 
we  concluded  were  due  to  very  simple  solvates,  such  as  are  formed  only 
in  the  most  concentrated  aqueous  solutions,  or  in  such  solutions  of  moder- 
ate concentration,  but  at  very  high  temperatures.  Donnan  and  Bassett 
assumed  that  these  bands  are  due  to  some  complex  anions,  such  as  C0CI2.CI 
or  C0CI2.CI2,  which  would  then  be  the  same  in  aqueous  and  non-aqueous 
solutions.  There  are  a  great  many  objections  to  this  explanation.  In  the 
first  place,  such  complexes  ought  to  obey  the  usual  rule  for  aggregates, 
that  is,  they  ought  to  break  down  with  rise  in  temperature,  whereas  the 
change  in  the  spectrum  demands  the  opposite.  In  the  second  place,  accord- 
ing to  this  theory,  the  bands  ought  probably  to  be  the  same  in  aqueous  as 
in  non-aqueous  solutions,  which  we  have  found  is  not  the  case.  On  the 
theory  of  solvates,  however,  everything  is  perfectly  clear.  The  difference  in  the 
structure  of  the  group  of  bands  with  different  solvents  is  what  we  should 
expect,  and  the  appearance  of  the  bands  with  rise  in  temperature  of  aque- 
ous solutions,  or  with  the  addition  of  large  quantities  of  a  dehydrating 
agent,  is  simply  due  to  the  formation  of  the  required  simple  hydrates 
under  these  conditions. 

The  bands  of  solutions  of  nickel  salts  are  all  of  the  same  type  as  the 
green  cobalt  band,  and  hence  must  be  studied  spectrophotometrically. 
The  change  in  the  ultra-violet  band  with  addition  of  dehydrating  agents, 
however,  suggests  that  here  also  hydrates  play  an  important  part.  An- 
hydrous nickel  chloride  could  not  be  dissolved  in  the  non-aqueous  solvents 
used,  hence  the  work  was  of  necessity  limited  to  aqueous  solutions. 

With  the  exception  of  copper  chloride  in  acetone,  which  has  a  band 
at  X  4700,  all  copper  solutions  show  only  two  regions  of  absorption,  one 
in  the  ultra-violet  and  one  in  the  red.  The  ultra-violet  band,  since  it 
narrows  rapidly  with  dilution  even  when  molecules  are  kept  constant, 
can  not  be  accounted  for  by  the  simple  theory  of  dissociation.  And  as  it 
widens  rapidly  with  rise  in  temperature,  we  must  conclude  that  it  is  due 


SUMMARY  AND    CONCLUSIONS.  103 

to  the  solvated  molecules,  the  absorbing  power  of  which  increases  rap- 
idly with  decrease  in  the  complexity  of  the  solvate. 

The  band  in  the  red  belongs  in  the  same  class  with  the  green  cobalt 
band.  But,  as  mentioned  above,  Miiller  came  to  the  conclusion  that  dis- 
sociation is  unable  to  account  for  its  deviation  from  Beer's  law,  which 
also  agrees  with  what  we  found  in  studying  its  behavior  in  mixtures  of 
alcohol  and  water  for  the  case  of  the  chloride.  Hence  we  assume  that 
solvates  here  also  play  a  r61e,  which,  however,  is  not  quite  so  apparent, 
owing  to  the  fact  that  both  the  solvated  ions  and  the  molecules  absorb 
light  in  this  region. 

Another  fact  which  supports  our  view  is  that  the  absorption  in  the  red  is 
not  widely  different  in  different  solvents,  provided  the  concentrations  are 
about  the  same;  while  in  the  ultra-violet  the  absorption  is  many  times 
greater  in  the  non-aqueous  than  in  the  aqueous  solvents;  the  reason  for  the 
latter  being,  first,  that  the  dissociation  in  aqueous  solutions  is  much  greater 
than  in  non-aqueous,  and  hence,  for  equal  concentrations,  the  number  of 
molecules  in  the  latter  is  much  greater  than  in  the  former.  Secondly,  the 
solvating  power  of  water  is  much  greater  than  that  of  the  non-aqueous  solv- 
ents used,  and  hence  the  comparatively  few  molecules  present,  by  forming  rel- 
atively complex  hydrates,  have  their  absorbing  power  still  further  reduced. 

The  only  salt  of  iron  studied  was  ferric  chloride.  It  shows  only  one 
region  of  absorption,  namely,  the  one  which  cuts  off  the  entire  ultra-violet, 
and  usually  also  the  violet  and  blue  portion  of  the  spectrum.  In  aqueous 
solutions  this  absorption  band  narrows  very  rapidly  with  dilution,  even 
when  molecules  are  kept  constant,  indicating  a  marked  effect  of  hydration. 
In  alcoholic  solutions  the  band  remains  of  sensibly  constant  width,  indicat- 
ing that  in  this  case  the  solvation  is  probably  very  slight.  The  diflBculty 
in  drawing  any  definite  conclusions  from  solutions  of  this  salt  is  that  the 
solutions  are  not  very  stable,  and  hence  the  effects  may  very  often  be 
marked  by  chemical  changes  of  unknown  amount. 

Chromium  salts  behave  very  much  like  those  of  nickel.  Only  two  of 
them  were  studied  in  this  work  and  these  only  in  aqueous  solution.  The 
behavior  of  their  bands  is  quite  analogous  to  that  of  the  green  cobalt  band, 
and  hence  calls  for  spectrophotometric  study.  Their  diffuse  character  also 
makes  them  rather  unfit  for  spectrographic  investigations. 

The  most  interesting  and  important  results  were  obtained  from  the 
study  of  the  salts  of  neodymium  and  praseodymium,  especially  those  of 
the  former.  These  substances  have  not  only  very  many  absorption  bands, 
but  they  are  remarkably  narrow  and  sharp,  and  hence  peculiarly  suitable 
for  spectrographic  study. 

The  chief  experimental  results  were  the  following: 

1.  The  absorption  spectrum  of  aqueous  solutions  of  the  chloride  and 
bromide  of  neodymium  changes  very  little  with  change  in  concentration, 
and  the  two  are  nearly  identical  throughout,  excepting  for  the  fact  that 
the  absorbing  power  of  the  bromide  appears  to  be  somewhat  greater  than 
that  of  the  chloride. 

2.  The  absorption  spectrum  of  aqueous  solutions  of  neodymium  nitrate 
is  somewhat  different  from  that  of  the  chloride  or  bromide,  especially  if 


104  ABSORPTION    SPECTRA    OF    SOLUTIONS. 

the  solutions  are  concentrated.  With  decrease  in  concentration  the  spec- 
trum changes  so  as  to  become  more  and  more  nearly  identical  with  that 
of  the  other  salts.  Certain  bands,  therefore,  increase  in  intensity  when  the 
conditions  for  Beer's  law  obtain.  Others  decrease  and  only  remain  constant 
when  molecules  are  kept  constant.  Examples  of  the  first  class  of  bands 
are  those  at  X  4275,  X  4330,  and  X  5205.  Examples  of  the  second  kind  are 
the  bands  at  X  4282  and  X  5235. 

3.  Addition  of  large  amounts  of  calcium  or  aluminium  chloride  to  a 
solution  of  neodymium  chloride  does  not  affect  the  spectrum  appreciably, 
except  the  shading  of  the  yellow  band,  and  perhaps  very  slightly  that  of 
the  green  bands. 

4.  Solutions  of  the  salts  in  non-aqueous  solvents  give  spectra  which 
are  not  only  different  for  different  salts,  but  the  spectrum  of  any  one  salt 
is  different  in  the  different  solvents.  An  apparent  exception  is  the  spectrum 
of  neodymium  or  praseodymium  chloride  in  methyl  and  ethyl  alcohols, 
which  are  almost  exactly  alike. 

5.  When  a  salt  like  neodymium  chloride  is  dissolved  in  mixtures  of 
water  and  one  of  the  non-aqueous  solvents,  and  the  relative  amount  of 
the  two  solvents  in  the  mixture  is  varied,  no  marked  change  in  the  spec- 
trum is  observed  when  the  amount  of  water  is  changed  from  100  per  cent 
to  about  15  or  20  per  cent.  As  the  amount  of  water  is  still  further  reduced, 
we  find  that  the  solution  gives  a  spectrum  which  consists  of  a  superposi- 
tion of  the  spectra  belonging  to  the  aqueous  and  the  non-aqueous  solu- 
tions, the  former  decreasing  in  intensity  while  the  latter  increases  as  the 
amount  of  water  is  decreased.  The  composition  of  the  mixed  solvent  which 
will  show  the  two  spectra  with  about  half  their  normal  intensity,  depends 
upon  the'concentration  of  the  salt  in  solution,  and  a  constant  ratio  between 
the  number  of  molecules  of  water  and  those  of  the  dissolved  salt  was  indi- 
cated by  the  experiments;  this  ratio  having  the  value  10. 

Neodymium  nitrate  dissolved  in  mixtures  of  water  and  one  of  the  non- 
aqueous solvents  shows  exactly  the  same  change  as  the  chloride;  but  in 
addition  we  get  the  changes  in  the  spectrum  produced  by  the  great  change 
in  the  state  of  dissociation  of  the  salt.  The  result  is  that  the  whole  change 
is  a  much  more  gradual  one,  and  hence  is  not  nearly  so  striking  as  it  is  in 
the  chloride  or  bromide  solutions. 

Praseodymium  chloride  dissolved  in  mixtures  of  water  and  methyl  or 
ethyl  alcohol  shows,  in  general,  the  same  kind  of  change  in  the  spectrum 
as  neodymium  chloride,  but  in  addition  there  appears  in  the  alcoholic 
solutions  an  entirely  new  band  having  no  analogue  in  the  aqueous  solu- 
tion. In  the  former  this  new  band  in  the  ultra-violet  is  by  far  the  most 
intense  in  the  entire  spectrum.  It  disappears  entirely  by  addition  of  water, 
having  about  half  its  normal  intensity  for  a  half-normal  solution  when 
the  water  content  of  the  solvent  is  about  8  per  cent. 

These  facts  seem  to  us  inexplicable  on  any  other  hypothesis  than  the 
one  we  have  advanced,  namely,  that  when  a  salt  of  one  of  these  elements 
is  dissolved  in  a  solvent  both  the  molecules  of  the  salt  and  the  ions  formed 
from  them  become  solvated,  that  is,  they  combine  with  a  certain  number 
of  molecules  of  the  solvent.     In  the  case  of  cobalt  and  copper  salts  we 


SUMMARY  AND    CONCLUSIONS. 


105 


found  reasons  for  believing  that  a  series  of  solvates  of  varying  complexity 
was  formed,  while  with  these  rare  elements  the  spectrum  rather  points  to 
the  existence  of  only  one  definite  solvate.  A  more  extended  study,  includ- 
ing the  changes  in  the  spectrum  produced  by  change  in  temperature,  may, 
however,  modify  this  conclusion  somewhat. 

Granting  the  existence  of  the  solvates,  the  phenomena  observed  in  the 
absorption  spectra  of  neodymium  and  praseodymium  admit  of  a  perfectly 
natural  explanation.  As  this  explanation  has  already  been  given  in  full 
under  the  discussion  of  neodymium  nitrate  in  water — Beer's  law — we  need 
not  repeat  any  part  of  it. 


I 


INDEX. 


Absorption  and  solvation 101 

spectra  of  solutions 1 

theories  of  absorption 101 

Acetone  and  water,  neodymium  nitrate 

in  mixtures  of 94 

Acetone,  cobalt  bromide  in — Beer's  law.  26 

chloride  in — Beer's  law.  18 

copper  chloride  in — Beer's  law.  48 

with  water  50 

ferric  chloride  in — Beer's  law.  62 

neodymiiun  nitrate  in — Beer's 

law 92 

with  water,  cobalt  bromide  in .  28 
chloride   in.   21 
Alcohols     and     water,     praseodymium 

chloride  in  mixtures  of  the 95 

Aluminiimi  and  calcium  chlorides — 

with  chromium  chloride 65 

with  neodymium  chloride  in  water .   77 

with  nickel  chloride  in  water 41 

Aluminium  chloride  with  ferric  chloride.  60 

Ames,  J.  S Preface 

Anderson,  on  the  absorption  spectra  of 

dry  neodymium  salts 85 

on  the  absorption  spectra  of 
powdered  salts  of  neodym- 
ium and  erbium 71 

Anhydrous  neodymium  chloride 84 

salts,  preparation  of 71 

Apparatus 6 

Association 2 

Babo,  work  on  cobalt  salts 11 

Bahr  and    Bunsen,   on  the  absorption 
spectra  of  compoimds  of  didymium. . .  68 

Bassett  and  Donnan 5 

assume  complex  ions 35 

on  the  absorption  of  cobalt  salts. . .  102 
on  the  absorption  spectra  of  copper 

salts 45 

on  the  color  changes  in  cobalt  salts.   13 
Becquerel,  absorption  spectra  of  chro- 
mium compoimds 63 

on  the  absorption  spectra  of 

didymium  compounds . .  68,  70 
on  the  absorption  spectra  of 

neodymium  salts 85 

Beer's  law 1 

Beer's  law  for — 

chromium  chloride  in  water 64 

nitrate  in  water 66 

cobalt  bromide  in  ethyl  alcohol 25 

in  methyl  alcohol . .   25 

in  water 22 

chloride  in  acetone 18 

in  ethyl  alcohol. ...   17 
^  in  methyl  alcohol . .   16 


Beer's  law  for — 

cobalt  chloride  in  water 13 

sulphate  in  water 31 

sulphocyanate  in  water. .  .32,  34 

copper  bromide  in  ethyl  alcohol ....  53 

in  methyl  alcohol ...   52 

in  water 51 

chloride  in  acetone 48 

in  ethyl  alcohol 47 

in  methyl  alcohol ...  46 

in  water 45 

nitrate  in  water 55 

erbium  chloride  in  water 97 

nitrate  in  water 97 

ferric  chloride  in  acetone .  62 

in  ethyl  alcohol 61 

in  methyl  alcohol . .  61 

in  water 59 

neodymium  bromide  in  water 86 

chloride  in  ethyl  alco- 
hol    78 

chloride  in  methyl  al- 
cohol    77 

chloride  in  water 72 

nitrate  in  acetone 92 

nitrate  in  ethyl  alco- 
hol    92 

nitrate  in  methyl  alco- 
hol    91 

nitrate  in  water 87 

nickel  acetate  in  water 43 

chloride  in  water 39 

sulphate  in  water 42 

praseodymium  chloride  in  water ...  94 

nitrate  in  water 96 

Bersch,  work  on  cobalt  salts 11 

Bettendorfif,  on  the  absorption  spectra 

of  the  rare  earths 68 

Boudouard,  on  the  absorption  spectra  of 

praseodymium  and  neodymiiun 68 

Brewster,  absorption    spectra   of  chro- 
mium salts 63 

on  the  absorption  spectrum  of 

nickel  nitrate 39 

Bunsen  and  Bahr,   on  the  absorption 

spectra  of  didymium  compounds 68 

Bunsen,  on  the  absorption  spectra  of 
didymium  salts 70 

Calcium  and  aluminium  chlorides — 

with  chromium  chloride 65 

with  neodymium  chloride  in  water .  77 

with  nickel  chloride  in  water 41 

Calcium  bromide  with  cobalt  bromide  . .  23 

chloride  with  ferric  chloride. ...  60 
Chatelier,  Le,  on  the  color  changes  in 

cobalt  salts 12 

107 


108 


INDEX. 


Chromium  chloride  in  water — Beer's  law.  64 
in   water — molecules 

constant 65 

with  calcium  and  alu- 
minium chlorides  .  65 
Chromium  nitrate  in  water — Beer's  law.  66 
in   water — ^molecules 

constant 67 

Chromiimi,  salts  of 63 

Cobalt  acetate  in  water — Beer's  law 34 

Cobalt  bromide  in — 

acetone — Beer's  law 26 

with  water 28 

ethyl  alcohol — Beer's  law 26 

with  water 27 

methyl  alcohol — Beer's  law 25 

with  water 27 

water — Beer's  law 22 

molecules  constant 22 

Cobalt  bromide  with  calciiun  bromide. .   23 
Cobalt  chloride  in — 

acetone — Beer's  law 18 

acetone  with  water 21 

ethyl  alcohol — Beer's  law 17 

ethyl  alcohol  with  water 20 

methyl  alcohol — Beer's  law 16 

methyl  alcohol  with  water 19 

water — Beer's  law 13 

water — ^ions  constant 15 

water — molecules  constant 15 

Cobalt  nitrate  in  water — ^molecules  con- 
stant    30 

Cobalt,  salts  of 11 

salts,  summary  of  results  with. . .  35 
sulphate  in  water — Beer's  law. . .  31 
Bulphocyanate  in  water — ^Beer's 

law 32 

sulphocyanate   in    water — ^mole- 
cules constant 33 

Conclusions  and  summary 100 

Copper  bromide  in — 

ethyl  alcohol  and  water 54 

— Beer's  law 53 

methyl  alcohol  and  water 54 

— Beer's  law 52 

water — Beer's  law 51 

molecules  constant 52 

Copper  chloride  in — 

acetone — Beer's  law 48 

with  water 50 

ethyl  alcohol — Beer's  law 47 

with  water 50 

methyl  alcohol — Beer's  law 46 

with  water 49 

water — Beer's  law 45 

molecules  constant 46 

Copper  nitrate  in  water — Beer's  law 55 

in  water — ^molecules  con- 
stant   66 

Copper,  salts  of 46 

Croft,  absorption  spectra  of  chromium 
compounds 63 

Demar^ay,  on  the  absorption  spectra  of 

didjrmium  compounds 68 

Dissociation 2 


Donnan  and  Bassett 5 

assume  complex  ions 35 

on  the  absorption  of  cobalt  salts 102 

on  the  absorption  spectra  of  copper 

salts 45 

on  the  color  changes  in  cobalt  salts.  13 
Drossbach,  absorption  spectra  of  praseo- 
dymium and  neodymium  compounds.  70 

Emsmann,  on  the  absorption  spectriun 

of  nickel  nitrate 39 

Engel,  on  the  color  changes  in  cobalt  salts  1 1 

Erbium  chloride  in  water — Beer's  law. .  97 

nitrate  in  water — Beer's  law. . .  97 

salts  of 68 

Etard,   on   the   absorption   spectra   of 

chromium  compounds 64 

on  the  color  changes  and  solu- 
bility of  cobalt  salts 11 

Ethyl  alcohol — 

and  water,  copper  bromide  in 54 

and  water,  neodymium  chloride  in .  83 

cobalt  bromide  in — Beer's  law 25 

cobalt  chloride  in — Beer's  law 17 

copper  bromide  in — Beer's  law. ...  53 

copper  chloride  in — Beer's  law 47 

ferric  chloride  in — Beer's  law 61 

neodymiiun  chloride  in — Beer's  law  78 

neodymiiun  nitrate  in — Beer's  law.  92 

with  water — cobalt  bromide  in 27 

with  water — cobalt  chloride  in 20 

with  water — copper  chloride  in 50 

Ferric  chloride — 

in  acetone — Beer's  law 62 

in  ethyl  alcohol — Beer's  law 61 

in  methyl  alcohol — Beer's  law 61 

in  water — Beer's  law 59 

in  water — molecules  constant 59 

with  aluminium  chloride 60 

with  calcium  chloride 60 

Gladstone,  absorption  spectra  of  chro- 
mium compounds 63 

Hartley,  absorption  spectra  of  chromi- 
imi compounds 63 

criticizes  the  views  of  Donnan 

and  Bassett 13 

on  change  in  absorption  with 
change  in  temperature 101 

on  the  absorption  spectra  of 

chromium  compounds 64 

on  the  absorption  spectra  of 

copper  salts 46 

on  tne  absorption  spectra  of 

salts  of  neodymium 70 

on  the  absorption  spectra  of 

solutions  of  cobalt  salts 12 

on  the  absorption  spectrum  of 

nickel  salts 39 

on  the  absorption  spectra  of 

the  nitrate  of  erbimn 70 

work  of 6 

Hydrate  of  cobalt  salts 36 

Hydrates  in  aqueous  solutions 1 


INDEX. 


109 


Introductory 1 

Ions  constant  for  cobalt  chloride  in  water  15 

for  nickel  chloride  in  water  40 

Iron,  salts  of 59 

Jones  and  Uhler — 

absorption    of    cobalt    chloride    in 

methyl  alcohol 17 

cadmium  zinc  spark 8 

on  dehydration 37 

on  solvation 38 

spectrograph 6 

work  on  cobalt  salts 11 

Jones,  on  the  absorption  spectra  of 
compounds  of  praseodymium  and 
neodymium 69 

Knoblauch,  absorption  spectra  of  chro- 
mium compounds 63 

on   the   absorption  spectra 

of  copper  salts 45 

Kriiss  and  Wilson,  on  the  absorption 
spectra  of  the  rare  earths 68 

Lapraik,  on  the  absorption  spectra  of 
chromium  compounds 64 

Light,  sources  of 8 

Liveing  and  Dewar,  absorption  spectra 
of  chromium  compounds 63 

Liveing,  on  the  effect  of  temperature  on 
the  absorption  spectra  of  compounds 
of  didymium  and  erbium 69 

Melde,  absorption  spectra  of  chromium 
compounds 63 

Methods  of  work 3 

Methyl  alcohol — 

and  water — copper  bromide  in  ... .  64 
and  water — neodymium  chloride  in 

mixtures  of 79 

and  water — ^neodymium  nitrate  in 

mixtures  of 93 

cobalt  bromide  in — Beer's  law 25 

cobalt  chloride  in — Beer's  law 16 

copper  bromide  in — Beer's  law 52 

copper  chloride  in — Beer's  law 46 

copper  chloride  in — with  water  ....  49 

ferric  chloride  in — Beer's  law 61 

neodymium  chloride  in — Beer's  law  77 
neodymiima  nitrate  in — Beer's  law.  91 

with  water,  cobalt  bromide  in 27 

with  water,  cobalt  chloride  in 19 

Miner,  H.  S Preface,  71 

Moissan,  absorption  spectra  of  chromi- 
lun  compovmds 63 

Molecules  constant  for — 

chromium  chloride  in  water 65 

chromium  nitrate  in  water 67 

cobalt  bromide  in  water 22 

cobalt  chloride  in  water 15 

cobalt  nitrate  in  water 30 

cobalt  sulphocyanate  in  water 33 

copper  bromide  in  water 52 

copper  chloride  in  water 46 

copper  nitrate  in  water 56 

ferric  chloride  in  water 59 


Molecules  constant  for — 

neodymium  chloride  in  water 76 

neodymium  nitrate  in  water 91 

nickel  chloride  in  water 41 

MuUer,  absorption  spectra  of  chromium 

compounds 63 

dissociation  can  not  account  for 

deviations  from  Beer's  law  . . .  103 
on  the  absorption  spectra  of  cop- 
per salts 45 

on    the    absorption    spectra    of 

nickel  and  copper  salts 100 

on  the  absorption  spectrum  of 

nickel  salts 39 

work  of 5 

Muthmann  and  Stiitzel,  on  the  absorp- 
tion spectra  of  salts  of  neodymivma. . .  70 
Muthmann,  on  the  absorption  spectra 
of  compounds  of  praseodymium  and 
neodymivmi 69 

Neodymium  bromide  in  water — Beer's 

law 86 

Neodymimn  chloride,  anhydrous 84 

Neodymium  chloride  in — 

ethyl  alcohol  and  water 83 

ethyl  alcohol — Beer's  law 78 

methyl  alcohol — Beer's  law 77 

mixtures    of    methyl    alcohol    and 

water 79 

water — Beer's  law 72 

water — molecules  constant 76 

water  with  calcium  and  aluminium 

chlorides 77 

Neodymium  nitrate  in — 

acetone — Beer's  law 92 

ethyl  alcohol — Beer's  law 92 

methyl  alcohol — Beer's  law 91 

mixtures  of  acetone  and  water 94 

mixtures    of    methyl    alcohol    and 

water 93 

water — Beer's  law 87 

water — molecules  constant 91 

Neodymium,  salts  of 68 

Nemst  filament 10 

lamp 9 

Nickel  acetate  in  water — Beer's  law 43 

Nickel  chloride  in  water — 

Beer's  law 39 

ions  constant 40 

molecules  constant 41 

with  calcium  and  aluminium  chlo- 
rides   41 

Nickel,  salts  of 39 

sulphate  in  water — Beer's  law. .  42 

Ostwald 1 

on  absorption  spectra 63 

on  the  color  of  solutions  of  cobalt 
salts 12 

Ostwald 's  theory  of  absorption  spectra. .  100 

Photographic  material 7 

Potilitzin,  on  the  color  changes  in  co- 
balt salts 11 


no 


INDEX. 


Praseodymium  chloride  in — 

mixtures  of  the  alcohols  and  water .  95 

water — Beer's  law 94 

Praseodymiimi  nitrate  in  water — Beer's 

law 96 

Praseodymium,  salts  of 68 

Pulfrich,  absorption  spectra  of  chromi- 
um compounds 63 

Rech,  on  the  absorption  spectra  of  aque- 
ous solutions  of  neodymium  chlonde.  71 
Recoura,  absorption  spectra  of  chromi- 
um compounds 63 

Results  obtained  in  this  work 103 

Rontgen  coil 9 

Russell,   on  the  absorption  spectra  of 
solutions  of  cobalt  salts 11 

Sabatier,  absorption  spectra  of  chromi- 
um compounds 63 

Schaeffer,  Helen 98 

on  the  absorption  spec- 
tra of  the  rare  earths.  70 
Scheele,   on  the  absorption  spectra  of 

praseodymivun  compoimds 68 

Schottlander,  on  the  absorption  spectra 

of  the  rare  earths _.  68 

Schunck,  absorption  spectra  of  chromi- 
um compounds 63 

Settegast,   absorption  spectra  of  chro- 
mium compounds 63 

Solutions,  absorption  spectra  of 1 

Solvation 2 

and  absorption 101 

Uhler  and  Jones 38 

Boret,  absorption  spectra  of  chromium 

compounds 63 

on  the  absorption  spectnma  of  nickel 

chloride 39 

Sources  of  light 8 

Spectra,  absorption,  of  solutions 1 

Spectrogram,  making  a 9 

Sttitzel  and  Muthmann,  on  the  absorp- 
tion spectra  of  salts  of  neodymium ...  70 


Summary  and  conclusions lOO' 

of  results  with  cobalt  salts ...  35 

Talbot,  absorption  spectra  of  chromium 

salts 63 

Temperature,  effect  of  on  absorption. . .  101 

Theories  of  absorption 101 

Tichbome,  on  the  color  changes  in  co- 
balt salts 11 

Uhler  and  Jones — 

absorption    of    cobalt    chloride    in 

methyl  alcohol 17 

cadmium  zinc  spark 8 

Carnegie  Publication  No.  60 

Preface,  1 

on  dehydration 37 

on  solvation 38 

spectrograph 6 

work  on  cobalt  salts 11 

Urbain,  on  the  separation  of  the  rare 

earths 70 

Vernon,  on  the  absorption  spectra  of 

chromium  compounds 64 

Vierordt,    absorption   spectra   of   chro- 

miima  compounds 63 

Vogel,  absorption  spectra  of  chromium 

compounds 63 

on  the  absorption  spectrum  of  nickel 
chloride 39 

Wainwright  and  Wratten 7 

Welsbach  Light  Co Preface,  71 

Wiedermann,  E.,  absorption  spectra  of 

chromium  compounds 63 

Wilson  and  Kruss,  on  the  absorption 

spectra  of  the  rare  earths 68 

Wratten  and  Wainwright 7 

Wyrouboff,    on    the   color   changes   in 

cobalt  salts 12 

Zimmermann,     absorption    spectra    of 
chromium  compounds 63 


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